Fixes annoying bug in tests.
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.

jhprinz/openpathsampling,jhprinz/openpathsampling,dwhswenson/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,dwhswenson/openpathsampling,openpathsampling/openpathsampling,choderalab/openpathsampling,openpathsampling/openpathsampling,jhprinz/openpathsampling