Verify expected units

[rkingsbury]

PR #6 addressed some inconsistencies w.r.t. the units of input kwargs vs. the units of the resulting simulation cell. However, the existing unit tests were still passing with the inconsistent units. After the fix, it was found that tests would not pass with density=1, whereas density=0.8 works.

To enhance robustness, we should add more comprehensive unit testing of the simulation cell size in test_generators.py to ensure that we are getting the expected size box from certain keyword arguments, and that tests fail when this is not the case.