Change CartesianPoints to be a Quantity subclass

closes #1368
update tests for new CartesianPoint behavior
updated tests and fixes based on tests
fixed all but one test failure
All tests now passing
fixed docs for Distance and CartesianPoint changes
Adjust CartesianPoint to duck typing instead of isinstance(x,Quanity)

as suggested by @mdboom
Merge pull request #1487 from eteq/cartesian-to-quantity

Make CartesianPoint and coordinate cartesian accessors Quantities