Support for other trajectory manipulation backends.

[blakeaw]

Currently PyBILT uses MDAnalysis as the backend for trajectory processing. It might be a nice upgrade to detach PyBILT from specifically needing MDAnalysis and allow it to use other trajectory analysis tools as the backend. The primary ones I am thinking of are that might be nice options are:

1. MDTraj http://mdtraj.org/1.9.0/
2. VMD http://www.ks.uiuc.edu/Research/vmd/current/ug/node160.html
3. pyLOOS http://loos.sourceforge.net/building.html#building_pyloos

Of the three listed above MDTraj would probably be the best to start with.