add desimodel files to datamodel

[sbailey]

The desimodel files are lightly documented in the desimodel/doc directory and in DESI-0847, but we should add more detailed data model documentation to desidatamodel. This would be a new top-level DESIMODEL directory, at the same level as DESI_SPECTRO_REDUX, etc.

[weaverba137]

In light of #7, we should really get these data models written.

[weaverba137]

The structure exists for this now, but no actual content has been added yet.

[weaverba137]

To support the 2017 Data Challenge, I'd like to get this done.

[weaverba137]

I'm starting to add files on the desimodel-update branch. Feel free to take a look at those and edit as needed.

[weaverba137]

At this point, I've added files that describe all areas of the desimodel data directory, except the spectra directory. While examining that directory, a few other questions came up that need to be answered before I can proceed (see desihub/desimodel#47, desihub/desimodel#48, desihub/desimodel#49) . In the meantime, you can look at and further edit the files on the desimodel-update branch.