Proposal assertion of units, in view of QMT/QMS refactor about qmt HOT 1 OPEN

Actually an input could also potentially be just a unit (would be a bit weird but lets's say someone just wanted 1eV, he can just put spu.eV which is of type unit rather than Mul. 1* spu.eV would be Mul because that's a multiplication).

In general I almost feel like trying to do type checking in python is a bit of a fool's errand (for e.g. if you assert that your input needs to be float, it could break if someone passes in a np.float64, but for all intents and purposes the input is correct and would have worked if not for the assertion). I think it would be sufficient to pass the input to to_float, and if it blows up we could catch that exception and give a more helpful error message, like "Input was not in the correct unit! expected unit is ..."

from qmt.