Use pooch instead of packaging STURM input files

[khaeru]

The Python source distribution (.tar.gz) created with python -m build is about 14 MB. Most of this is the STURM .RData input files; with these removed the archive is only about 3.6 MB.

This is exactly one intended application of the pooch package; see “Manage a package's sample data” in its documentation.

To do:

• Add a module that uses Pooch to download the STURM input data files.
• Show in the documentation how to use these.
• Exclude the files from the source distribution.