[material][SimpleCO2] fix enthalpy calculation

What this MR does / why does DuMux need it:

Fixes some errors related to the SimpleCO2 component.

We should add a test that uses the nonisothermal aspects of this component. The BrineCO2 fluidsystem is not well suited for it, as it uses liquidX functions in strange places. As that fluidsystem was created for supercritical conditions it's not really a good fit for the SimpleCO2 component anyway.

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