Add broadcast area model, loading from GeoJSON

This commit adds a new model class which can be used by any app to
interact with a broadcast area. A broadcast area is one or more polygons
representing geographical areas.

It also adds some models that make browsing collections of these areas
more straightforward. So the hierarchy looks like:

> **BroadcastAreaLibraries*
> Contains multiple libraries of broadcast area
> > **BroadcastAreaLibrary**
> > A collection of geographic areas, all of the same type, for example
> > counties or electoral wards
> > **BroadcastArea**
> > Contains one or more shapes that make up an area, for example
> > England
> > > **BroadcastArea.polygons[n]**
> > > A single shape, for example the Isle of Wight or Lindisfarne
> > > > **BroadcastArea.polygons[n][o]**
> > > > A single coordinate along a polygons

The classes support iteration, so all the areas in a library can be
looped over, for example if `countries` is an instance of
`BroadcastAreaLibrary` you can do:
```python
for country in countries:
    print(country.name)
```

The `BroadcastAreaLibraries` class also provides some useful methods for
quickly getting the polygons for an area or areas, for example to
render them on a map. So if `libraries` is an instance of
`BroadcastAreaLibraries` you can do:
```python
libraries.get_polygons_for_areas_long_lat('england', 'wales')
```

This will give polygons for the Welsh mainland, the Isle of Wight,
Anglesey, etc.

The models load data from GeoJSON files, which is an open standard for
serialising geographic data. I’ve added a few example files taken from
http://geoportal.statistics.gov.uk to show how it works.

notifications_utils/broadcast_areas/__init__.py

@@ -0,0 +1,168 @@
+import itertools
+from contextlib import suppress
+from pathlib import Path
+from functools import lru_cache
+import geojson
+from notifications_utils.formatters import formatted_list
+
+from notifications_utils.serialised_model import SerialisedModelCollection
+from notifications_utils.safe_string import make_string_safe_for_id
+
+
+@lru_cache(maxsize=128)
+def load_geojson_file(filename):
+
+    path = Path(__file__).resolve().parent / filename
+
+    geojson_data = geojson.loads(path.read_text())
+
+    if not isinstance(geojson_data, geojson.GeoJSON) or not geojson_data.is_valid:
+        raise ValueError(
+            f'Contents of {path} are not valid GeoJSON'
+        )
+
+    return path.stem, geojson_data
+
+
+class IdFromNameMixin:
+
+    @property
+    def id(self):
+        return make_string_safe_for_id(self.name)
+
+    def __repr__(self):
+        return f'{self.__class__.__name__}(<{self.id}>)'
+
+    def __lt__(self, other):
+        # Implementing __lt__ means any classes inheriting from this
+        # method are sortable
+        return self.id < other.id
+
+
+class GetItemByIdMixin:
+    def get(self, id):
+        for item in self:
+            if item.id == id:
+                return item
+        raise KeyError(id)
+
+
+class BroadcastArea(IdFromNameMixin):
+
+    def __init__(self, feature):
+        self.feature = feature
+
+        for coordinates in self.polygons:
+            if coordinates[0] != coordinates[-1]:
+                # The CAP XML format requires shapes to be closed
+                raise ValueError(
+                    f'Area {self.name} is not a closed shape '
+                    f'({coordinates[0]}, {coordinates[-1]})'
+                )
+
+    def __eq__(self, other):
+        return self.id == other.id
+
+    @property
+    def name(self):
+        for possible_name_key in {
+            'rgn18nm', 'ctyua16nm', 'ctry19nm',
+        }:
+            with suppress(KeyError):
+                return self.feature['properties'][possible_name_key]
+
+        raise KeyError(f'No name found in {self.feature["properties"]}')
+
+    @property
+    def polygons(self):
+        if self.feature['geometry']['type'] == 'MultiPolygon':
+            return [
+                polygons[0]
+                for polygons in self.feature['geometry']['coordinates']
+            ]
+        if self.feature['geometry']['type'] == 'Polygon':
+            return [
+                self.feature['geometry']['coordinates'][0]
+            ]
+        raise TypeError(
+            f'Unknown geometry type {self.feature["geometry"]["type"]} '
+            f'in {self.__class__.__name} {self.name}'
+        )
+
+    @property
+    def unenclosed_polygons(self):
+        # Some mapping tools require shapes to be unenclosed, i.e. the
+        # last point joins the first point implicitly
+        return [
+            coordinates[:-1] for coordinates in self.polygons
+        ]
+
+
+class BroadcastAreaLibrary(SerialisedModelCollection, IdFromNameMixin, GetItemByIdMixin):
+
+    model = BroadcastArea
+
+    def __init__(self, filename):
+        self.name, geojson_data = load_geojson_file(filename)
+        self.items = geojson_data['features']
+
+    def get_examples(self, max_displayed=4):
+
+        truncate_at = max_displayed - 1
+
+        names = [area.name for area in sorted(self)]
+        count_of_excess_names = len(names) - truncate_at
+
+        if count_of_excess_names > 1:
+            names = names[:truncate_at] + [f'{count_of_excess_names} more…']
+
+        return formatted_list(names, before_each='', after_each='')
+
+
+class BroadcastAreaLibraries(SerialisedModelCollection, GetItemByIdMixin):
+
+    model = BroadcastAreaLibrary
+
+    def __init__(self):
+
+        self.items = list(
+            Path(__file__).resolve().parent.glob('*.geojson')
+        )
+
+        self.all_areas = list(self.get_all_areas())
+
+        seen_area_ids = set()
+
+        for area_id in (area.id for area in self.all_areas):
+            if area_id in seen_area_ids:
+                raise ValueError(
+                    f'{area_id} found more than once in '
+                    f'{self.__class__.__name__}'
+                )
+            seen_area_ids.add(area_id)
+
+    def get_all_areas(self):
+        for library in self:
+            for area in library:
+                yield area
+
+    def get_areas(self, *area_ids):
+        return list(itertools.chain(*(
+            [area for area in self.all_areas if area.id == area_id]
+            for area_id in area_ids
+        )))
+
+    def get_polygons_for_areas_long_lat(self, *area_ids):
+        return list(itertools.chain(*(
+            area.polygons
+            for area in self.get_areas(*area_ids)
+        )))
+
+    def get_polygons_for_areas_lat_long(self, *area_ids):
+        return [
+            [[long, lat] for lat, long in polygon]
+            for polygon in self.get_polygons_for_areas_long_lat(*area_ids)
+        ]
+
+
+broadcast_area_libraries = BroadcastAreaLibraries()