Switched to can_append/can_prepend.

Function names, tests. Removed legacy `.forward` and `.backward` code.
Switch ala_dipeptide_example to `can_append`
Use can_append in toy_dynamics_tis.ipynb.

Also removed the .ipynb_checkpoints/ directory from version control
Shooting movers now use can_append/can_prepend.
Attic'd interface.py.

Interfaces are defined by volumes in volume.py; ensembles for the interfaces
are defined in ensemble.py.
Ensemble.locate => Ensemble.find_valid_slices
Changes to sandbox. But sandbox appears broken...

It looks like some change to the storage broke the sandbox. It would have
been broken anyway before I made the imports absolute in this commit, so I'm
not going to try to fix it with this PR.
Trajectory.forward => .append; attic sandbox
fixed correct registration of saved OPs
Removed frames attribute from VolumeEnsembles.

It was mostly unused, and the implementations were terribly broken anyway.
The better way to do this would be to implement an AlteredTrajectoryEnsemble
subclass that handles slices. Note that there may still be similar things to
clean up in LengthEnsemble.
added Alanine Example
Draft of OptionalEnsemble
Draft of SlicedTrajectoryEnsemble
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.
AlteredTrajectoryEnsemble => AlteredEnsemble
Merge branch 'altered_ensemble' into optional_ensemble
Make OptionalEnsemble into an AlteredEnsemble.
Merge branch 'master' of github.com:choderalab/msm-tis

Conflicts:
	examples/ipython/Visualization Example.ipynb
Merge remote-tracking branch 'upstream/master' into annoying_test_output_fix
Added note to self for future improvements