Feature suggestion: Write out physical units used in .data files

[hamogu]

Some of MESA's output is in css units, but some columns are in solar units, and some are unitless (e.g. abundance ratios).
It would be useful for users of MESA output, particularly for people who did not run the simulations by themselves, but got the model output from a collaborator or from a paper or database, to know the units at least for columns put out by the code itself (I know users can add even more columns to the hundreds that they can already activate).

How could that be implemented with minimal disruption? I'm not a regular MESA user (but just someone who occasionally gets files and then has to go back my collaborator who generated them or to the database to figure out what the units are), so I don't really know. Maybe you could put extra line below the column names in the .data files (maybe with a comment symbol)? I don't know how much that would disrupt regular users.

Several spec for writing units in ASCII symbols in astronomy exist and it would be best to adopt one of the widely used ones, i.e. the IAU Style manual that is used in fits headers or the CDS table format.

[earlbellinger]

I agree this would be both useful and disruptive (would break all parsing scripts).

FWIW the units can be found here: https://github.com/MESAHub/mesa/blob/main/star/defaults/history_columns.list

[VincentVanlaer]

I think this would be possible for HDF5 based outputs (see #748). You would be able to store the units in attributes.

[warrickball]

We could definitely add it to HDF5 data as attributes, if it existed.

Perhaps we could render the history_columns.list and profile_columns.list in the docs somewhere (probably the reference section)? It's probably not too difficult to be able to get lines like

...
!# nuclear reactions

   !## integrated quantities

      !power_h_burn ! total thermal power from PP and CNO, excluding neutrinos (in Lsun units)
      !power_he_burn ! total thermal power from triple-alpha, excluding neutrinos (in Lsun units)
...

to render as

nuclear reactions

integrated quantities

name	description
power_h_burn	total thermal power from PP and CNO, excluding neutrinos (in Lsun units)
power_he_burn	total thermal power from triple-alpha, excluding neutrinos (in Lsun units)

where here I'm a bit limited by GitHub Markdown (I have to have headers) and I haven't modified the source style at all. We'd have to tweak some entries to make it work and multi-line comments are a bit more complicated but I think it's all workable.