Fix Thermo conversion unit tests.
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)
enochd/RMG-Py,faribas/RMG-Py,faribas/RMG-Py,KEHANG/RMG-Py,comocheng/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,nickvandewiele/RMG-Py,KEHANG/RMG-Py,comocheng/RMG-Py,enochd/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py