Add Celcius and Fahrenheit

Why are Celcius and Fahrenheit not in `units`? They will only take 2 minutes to add... BUT THEN...

This proves to be more difficult than originally thought because `CompositeUnit` does not support addition or subtraction, only scaling. So for now, the conversion actually needs special equivalencies.

I have not added any change log or tests because I do not know if this is the best solution, or if changing `CompositeUnit` to support this is worth the trouble.

I open this PR anyway for discussions.