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:

wrappers/python/simtk/unit/doctests.py

@@ -625,9 +625,9 @@ Examples
 
         >>> 1.2*meters < 72*centimeters
         False
-        >>> meter != None
+        >>> meter is not None
         True
-        >>> meter == None
+        >>> meter is None
         False
 
         Examples

wrappers/python/simtk/unit/mymatrix.py

@@ -60,7 +60,7 @@ def zeros(m, n=None):
      [0, 0, 0]
      [0, 0, 0]]
     """
-    if n == None:
+    if n is None:
         n = m
     result = []
     for row in range(0, m):

wrappers/python/simtk/unit/quantity.py

@@ -114,7 +114,7 @@ class Quantity(object):
          - unit: (Unit) the physical unit, e.g. simtk.unit.meters.
         """
         # When no unit is specified, bend over backwards to handle all one-argument possibilities
-        if unit == None: # one argument version, copied from UList
+        if unit is None: # one argument version, copied from UList
             if is_unit(value):
                 # Unit argument creates an empty list with that unit attached
                 unit = value
@@ -167,7 +167,7 @@ class Quantity(object):
             value = value * unit._value
             unit = unit.unit
         # Use empty list for unspecified values
-        if value == None:
+        if value is None:
             value = []
 
         self._value = value
@@ -306,7 +306,7 @@ class Quantity(object):
             value_factor = 1.0
             canonical_units = {} # dict of dimensionTuple: (Base/ScaledUnit, exponent)
             # Bias result toward guide units
-            if guide_unit != None:
+            if guide_unit is not None:
                 for u, exponent in guide_unit.iter_base_or_scaled_units():
                     d = u.get_dimension_tuple()
                     if d not in canonical_units: