Fix small bug in unit related to mutable sequences.
Fix the mean() and std() methods on container Quantities when those containers
are NOT numpy arrays.
Add a set of unittests for the simtk.unit package. Implements most of what I
found in doctests.py (and quite a bit more, too)
chemistry.unit -> simtk.unit
Merge pull request #750 from swails/fix_unit

Fix small bug in `simtk.unit` related to mutable sequences.