Show failing case where subtract of zero loses Unit class.

Currently, if you subtract a zero unit from another, it will revert
to the scalar class, rather than maintain its Unit class. However
for addition, it is still respected.
Fix the failing cases of subtraction

This fix caters for the loss of Units class when two zero Units
are subtracted.
Merge pull request #61 from agworld/fix_subtract_class

Fix failing case of subtraction losing Units class