never axiomatize using 'weight', always use 'mass'

[cmungall]

we have distinct terms for body mass and body weight. Technically these are distinct, by their axiomatization - the body mass will change on the moon due to reduced gravity.

Obviously whenever we say "weight" in a trait context, we mean "mass". We should have a check to make sure weight is never used. We may also want to add a warning to PATO too.