cupSODA: Create cupSODA unit tests

[alubbock]

Issue from https://github.com/LoLab-VU/pysb-cupsoda/issues/3 originally by @lh64:

Create some unit tests

[JamesPino]

Add tests for setting vol (need to add that functionality as well)

[JamesPino]

Check coverage to make sure everything is above 90%