Change the way Wilhoit class stores/calculates E0

The constant of integration H0 is not equal to the enthlapy at 0K,
so the parameter E0 has to be calculated instead of just returning H0.
Fix Thermo conversion unit tests.

Some of the constants in the setup were inconsistent. I think these are now right.
I also added two "round trip" conversion tests, that go from Wilhoit back to Wilhoit
either via NASA or ThermoData. These now all pass.
(I had to loosen the "almost" equal tolerance for E0, but I think 10 or 20 J/mol is ok)