In __mul__, __div__, convert strings to units before anything else
For units, only instantiate Quantity(1, unit), not Quantity(other, unit)
Now also simplified the Quantity side, letting Unit take care of strings
Remove xfail on test resolved by #1557
Change test for pickling to ensure resulting units are equivalent
Merge pull request #1558 from mhvk/units-quantity-interaction-simplification

Units-quantity interaction simplification