Fix Wilhoit unit tests for new E0 behavior.
Use E0 on new Wilhoit class to set Conformer energy.
Increase limit for warning about poor heat capacity fits.

This is not a very helpful warning.
How high is too high? What impact does it have? What can we do about it anyway?
Don't use Conformers just to store E0 from the Thermo data.

Rather than create a Conformer as a side effect of getting thermo data,
we now just make sure the thermo data has an E0 attached to it
(this requires fitting a Wilhoit if, for example, you read a NASA polynomial
in from a database, and want to keep it as a NASA polynomial).

We now only create Conformers if and when needed in the generateStatMech()
method rather than the generateThermoData() method.