Add functions to Quantity to compute the max, min, standard deviation (as the
sqrt of the variance), and average, returning a Quantity with the proper units.

This should be reasonably efficient, as it takes advantage of numpy-accelerated
methods if they're present.
More stable std calc.
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:
Possible speedup for unit_math.sum function.
Merge pull request #643 from swails/numpy-unit

[WIP] Add functions to Quantity to compute the max, min, standard deviation and average of a Quantity, returning a Quantity.