Add the Eos function to convert CT into PotT

[alicebarthel]

This is a small PR to introduce the EOS helper function (CT-->PT, i.e. calcPtFromCt) (and the calcCtFromPt, calcGibbsDerivPt0Pt0 helper functions that it requires).
These conversion functions are not currently plugged into anything but will be required in the coupling steps later.
Since this is an inner helper function, I only added a line in the devGuide to inform of the capability.
A couple of Ctests simply compare a single-cell value between the GSW-C module and our implementation of calcPtFromCt and calcCtFromPt.

I also made modifications to PhysicalConstants to introduce variables for future coupling, namely Cp0Sw, SS0, Psu2Gpkg and Sfac, which are not currently used anywhere.

Checklist

• Documentation:
  • User's Guide has been updated
  • Developer's Guide has been updated
  • Documentation has been built locally and changes look as expected
• Linting
  • Pre-commit run locally
• Building
  • CMake build does not produce any new warnings from changes in this PR
• Testing
  • Ctests all passed on PM-CPU (gnu), PM-GPU
  • The Polaris omega_pr test suite
    has passed, using the Polaris e3sm_submodules/Omega baseline
  • New tests:
    • CTest unit tests for new features have been added per the approved design.