Changing what ZPE is read for compound methods

This commit changes what ZPE is read in for compound
methods like CBS-QB3 and G3.

Cantherm now reads in
E (0 K) which is E_elec + scaled ZPE
It then calculates E_elec = E (0 K) - E(ZPE)

Later it calculates the ground state energy as
E0 = E_elec + unscaled ZPE * scaling factor from input file.

This is done so to be consistent when we use different method/basis
for energy and frequency calculations (ex F12 jobs). It also
puts the onus on the user to suply the correct scaling factor for
the method.

Note: The loadZeroPointEnergy() still reads in the unscaled ZPE

closes issue #132
Merge branch 'master' of github.com:GreenGroup/RMG-Py