Data model for GFA summary files

[weaverba137]

This PR closes #201.

We still need to fill in the details of the stub data model file, so please do not merge yet.

[coveralls]

coverage: 100.0%. remained the same
when pulling 42be390 on gfa-summary
into 6684057 on main.

[weaverba137]

Also note: ensure that EXTNAME is set for all HDUs: desihub/gfa_reduce#26.

[araichoor]

for reference, this page hopefully has description of most of the columns: https://desi.lbl.gov/trac/wiki/SurveyValidation/SV1/conditions/summary_files.

[weaverba137]

Thank you @araichoor, I'll try to fill in as much from there as possible.

[sbailey]

Checking in on the status of this. It looks like the EXPOSURE_SUMMARY and EXPOSURE_SUMMARY_STRICT HDUs are well documented, but there are many missing descriptions for CAMERA_SUMMARY, which includes the caveat "... most of which shouldn't be relevant for the purpose of estimating effective exposure times." It looks like many of them could be filled in by extrapolating from other columns, e.g. MAX_Q1 does have a description, from which we could make a reasonable guess of what MIN_Q1, MEAN_Q1, and MEAN_QA also mean.

In the spirit of not letting the perfect be the enemy of the good, let's either wrap this up in the next week or so, or merge it as-is and save further documentation for if/when we really need it.

[weaverba137]

@ameisner, in the CAMERA_SUMMARY HDU, do the columns come from header cards in another file? If so, do those header cards have comments? If so, we could just grab the descriptions from those comments. It's just a matter of which file contains these headers.