Pressure unit mbar doesn't function in RMG, and a general units question

[alongd]

The mbar pressure unit appears in commonUnits in quantity.py, but it doesn't get converted into Pa in main.py.
This unit is not mandatory, since RMG's existing pressure units already cover the realistic pressure range without using too many zeros. However, including it will avoid an unnecessary job crash (I got: LookupError: Unable to parse units: "mbar").

Perhaps more important, I couldn't find in the documentation (i.e., without looking at the code) which units are allowed for frequently used dimensions in RMG. I guess including this in the documentation could be helpful for users? (or have I missed it?)

[github-actions]

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