Fix another small bug for unit handling with numpy arrays.  Fixes the exception:

Traceback (most recent call last):
  File ".../python/tests/TestNumpyCompatibility.py", line 87, in testNumpyAttributes
    d = self.data.reshape((100, 3))
  File ".../simtk/unit/quantity.py", line 575, in reshape
    return Quantity(self._value.reshape(shape, order=order))
  File ".../simtk/unit/quantity.py", line 142, in __init__
    if value == first_item:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

Also add a test for new unit numpy capabilities.