ensure coerce only applies to multiplication

@fotos Intuitively, I would perfectly accept "(2) * (3 meters)" as "6 meters", whereas "2 + (3 meters)" doesn't have matching units. Notice that "(2) * (3 meters)" means "(3 meters) + (3 meters)" which does indeed have matching units.

Random somewhat relevant link: http://www.chem.ox.ac.uk/teaching/Physics%20for%20CHemists/Units.html