Fix the mean() and std() methods on container Quantities when those containers

are NOT numpy arrays.

wrappers/python/simtk/unit/quantity.py

@@ -506,7 +506,7 @@ class Quantity(object):
         except AttributeError:
             if args or kwargs:
                 raise TypeError('Unsupported arguments for Quantity.mean')
-            mean = self.sum() / len(self._value)
+            mean = (self.sum() / len(self._value))._value
         return Quantity(mean, self.unit)
 
     def std(self, *args, **kwargs):
@@ -526,7 +526,8 @@ class Quantity(object):
         except AttributeError:
             if args or kwargs:
                 raise TypeError('Unsupported arguments for Quantity.std')
-            mean = self.mean()
+            mean = self.mean()._value
+            var = 0
             for val in self._value:
                 res = mean - val
                 var += res * res