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

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](https://iauarchive.eso.org/publications/proceedings_rules/units/) that is used in fits headers or the [CDS table format](https://vizier.unistra.fr/doc/catstd-3.2.htx).

Start by locating the code that writes MESA .data files and the definitions of their output columns. Review the IAU Style manual and CDS table format references cited in the issue, then assess a minimally disruptive way to emit units for code-generated columns. Done means the units are consistently written without disrupting existing users.

Written by the indexing model from the issue text.