MEASURE.copy() now properly handles when attributes are None.
MEASURE.copy() now properly handles when attributes are None.

Passing None to a Quantity() constructor does *not* return None, but
instead sets the value to zero and the units to dimensionless. Clearly
zero is not a copy of None, so we need to check for None and handle it
as a special case.

enochd/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py,pierrelb/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,KEHANG/RMG-Py,KEHANG/RMG-Py,enochd/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py