Make MOPAC's PM3 and PM6 classes reuse code (and add PM7)

I think we went with a Class-based system of inheritance, so that you could override
arbitrary methods if necessary. Eg. you could have a Class B3LYP that would 
have to do bond additive corrections, or atomization energies, or something,
that would require additional methods and functions. 

For now, however, most methods differ just in one or two key words, so it seems
overkill. Should be extensible though...