Bug fix for internal mode projection for Cantherm

We were not following the Gaussian white paper formulae and this resulted
in some cases where our projection was completely wrong.

This bug fix has been provided by @agvandeputte
Correcting the ZPE after projection

After projecting the hindered rotors we need to recalculate the ZPE
according to the projected frequencies. Since  'ideally" only the modes with
a small wave number are projected out this should not be that different from
the ZPE coming before projections.