resolve issue 3843: fix quart constant value in physics.units

There are 231 cubic inches in a US gallon, not in a US quart as is currently defined.    This appeared in a code example in the docstring for the physics.units module, so I've updated the docstring, as well as the module variable initializer for quart in the attached file.

Additionally, i've added a test function to test_units.py to verify example code in docstring