Delete now out-of-date docs as well as adding __array_priority__ to Quantity.

This should be *lower* than that of Unit (and ScaledUnit/BaseUnit), since we
still want Units to take priority over Quantities (but Quantity to take
precedence over regular ndarrays) for reasons like this:

>>> a = np.array([1]) * u.angstroms
>>> b = a * u.angstroms # should be np.array([1])*u.angstroms**2
>>> np.arange(10) * a # utilize broadcasting
[1 2 3 4 5 6 7 8 9 10] A**2

docs-source/usersguide/library.rst

@@ -2712,10 +2712,6 @@ multiply operator (‘*’) or the explicit Quantity constructor:
     # or more verbosely
     bond_length = Quantity(value=1.53, unit=nanometer)
 
-When working with Numpy arrays you *must* use the explicit constructor.  You cannot
-multiply them by a unit, because the Numpy array class overloads the multiply operator.
-
-
 Arithmetic with units
 ---------------------
 

wrappers/python/simtk/unit/quantity.py

@@ -92,6 +92,7 @@ class Quantity(object):
       4 - lists of tuples of numbers, lists of lists of ... etc. of numbers
       5 - numpy.arrays
     """
+    __array_priority__ = 99
 
     def __init__(self, value=None, unit=None):
         """