Stop using "== None" and "!= None". "is" and "is not" is the preferred way to do
it.  Not only that, I got this FutureWarning:

/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/simtk/unit/quantity.py:170:
FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  if value == None: