test data not included in installable package

[orbeckst]

Tests cannot be run from an installed package:

pip install mdpow
pytest --pyargs mdpow   # <--- FAILS

The files under mdpow/tests/testing_resources/ are not included as data files in the package.

[orbeckst]

Including the testing resources increases the package size from 3.1M to 23M.

[orbeckst]

Following https://setuptools.pypa.io/en/latest/userguide/datafiles.html we could include data files in the tar.gz but exclude from the wheel.

[orbeckst]

Note that pip installing from sdist builds a wheel and that excludes tests so it's still not possible to run

pytest --pyargs mdpow

For right now, must run tests with sources.