Initial work toward Exit/EnterXEnsembles.

Also, corrects a few points on volume, and adds skeletons of the test
classes for these ensembles.
Steps toward implementing ensemble tests.
Tests for Enters/ExitsXEnsemble.

Pretty setup, with a few clever hacks to make tests more manageable.
Exits/EntersXEnsemble. Code passes tests.
gitignore inside src.

For future reorg; ignores *pyc and tags.
Merge remote-tracking branch 'origin/master' into enter_exit_ensembles
Merge remote-tracking branch 'upstream/master' into enter_exit_ensembles

Conflicts:
	code/python/PySRMSTIS/src/ensemble.py
Merge remote-tracking branch 'upstream/master' into enter_exit_ensembles
Got rid of mdtraj.pdb
Merge pull request #30 from dwhswenson/enter_exit_ensembles

ExitsXEnsemble/EntersXEnsemble