Add option to use mbar when using physical temperatures

[pgrete]

Migrated from GitLab: https://gitlab.com/theias/hpc/jmstone/athena-parthenon/athenapk/-/issues/8:

In the future, mbar might be the more generalizable parameter for defining the temperature. We could add it as an alternative to defining He_mass_fraction.
However, since He_mass_fraction is used elsewhere during the cluster setup, we can leave it as it for now.

[BenWibking]

This would also be useful for my problem generator.