Allow generation of Fortran include or module file from NIST codata

[awvwgk]

Is your feature request related to a problem? Please describe.

The NIST codata available in qcel can currently be exported to a C header, it would be interesting to have an export to a Fortran include or module file as well.

Describe the solution you'd like

Have an additional function in https://github.com/MolSSI/QCElemental/blob/master/qcelemental/physical_constants/context.py to export a Fortran include or module file.

Describe alternatives you've considered

The C header could be included in Fortran as well since it only contains macro definitions.

Additional context

Discussion on parsing NIST codata on the Fortran discourse: https://fortran-lang.discourse.group/t/tips-to-make-this-toy-program-faster/803/38

[loriab]

Generating a Fortran header would be a welcome addition. That C header is tailored for Psi4, and the idea was any number of header exporters could be added to bridge btwn the qcel representation and whatever downstream programs need.

[awvwgk]

Is there a CI check in this repository of the C header we could replicate in Fortran?

[loriab]

Only that it writes, not that it roundtrips. https://github.com/MolSSI/QCElemental/blob/master/qcelemental/tests/test_constants.py#L106-L110

Diff with the existing header has been the sanity/format check. Admittedly not very strong w/o an existing downstream.