Wilhoit thermo enforces constant Cp for monoatomic species

[rwest]

Things like O atom will (probably?) have incorrect thermo once they have been converted to Wilhoit form (and then onwards to NASA). Wilhoit uses the Cp0 and Cp_inf limits, which will both be 2.5*R for a monoatomic species.

Somebody should check our element atom enthalpy curves. (C, H, O, etc.)

[github-actions]

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[rwest]

I think not a problem