Use built in wilhoit property E0 for conformer E0 in model.

Wilhoit's get E0 function actually uses wilhoit.getEnthalpy(.001) which is
slightly different than wilhoit.getEnthalpy(1.0).  So I am changing it
for consistency.  Hopefully this is the only instance of this.
Save the E0's in thermo if NASA polynomial original is saved and didn't have one.

It is necessary for calculating barrier heights in reactions.