`.xlsm` files are like `.xlxs` files but with macros enabled. They store
data in the same XML-based format as `.xlsx` files.
Pyexcel will try to use the xlrd package to parse `.xlsm` files. This
used to work because xlrd used to support reading `.xlsx` files. xlrd
has dropped support for `.xlsx` files in version 2 because of security
concerns. This means that when pyexcel asks xlrd to parse a `.xlsm` file
it causes an error.
This commit adds some branching to force `.xlsm` files to be opened
with pyexcel-xlsx instead, which does support `.xlsx` files.
These are no longer used anywhere. Note that pyexcel-xlsx is still
a necessary dependency: it acts as an implicit plugin to pyexcel
that, surprisingly, allows pyexcel to...read excel files [1].
[1]: https://github.com/pyexcel/pyexcel-xlsx#as-a-pyexcel-plugin
`app/utils.py` is a bit of a dumping ground for things we don’t have a
better place for.
We now have a place and structure for storing ‘model’ code (‘model’ in
the model, view, controller (MVC) sense of the word).
This commit moves the spreadsheet model to that place.
