Add the E0 property to all Thermo (HeatCapacityModel) classes.

It is now in the construction signature of these classes where it should 
be specified (NASA, and ThermoData). For Wilhoit it is the same as H0 and
so should not be specified (it is calculated when you fit the Wilhoit. If you wish
to specify manually, change H0).
Added E0 to Thermo unit tests.

Many work, but the conversion between NASA/Wilhoit/ThermoData is poor to say the least.
E0 varies by hundreds of kJ/mol!