Fixes annoying bug in tests.

When running tests, we'd hit lines that JHP had put in warning that a
snapshot should never be without velocity/kinetic energy. However, this
*does* happen when loaded a traj from a PDB, which doesn't have those. And
since we do that in tests.setup, it was spitting out warnings every time I
ran a test.
Merge remote-tracking branch 'upstream/master' into annoying_test_output_fix
Added note to self for future improvements
Merge pull request #77 from dwhswenson/annoying_test_output_fix

Annoying test output fix
Merge remote-tracking branch 'upstream/master' into ensemble_cleanup