Fixed some import errors and moved harmonic_oscillators_update.py

Fixes:

1.  Broken import statement in docstring
2.  Moved harmonic_osciillators_update.py to a good location.  This file was a standalone script and did not belong inside the pymbar library directory.  This may have also been preventing docstrings from running properly via nosetests, which I don't really understand--moving the files seems to fix it.