don't rely on docDB to document fiberassign data model

[moustakas]

In order to properly interpret the fiberassign HDUs --
https://desidatamodel.readthedocs.io/en/latest/DESI_TARGET/fiberassign/tile.html
one has to read DESI-0530 and DESI-1049 on DocDB.

It would be nice to have the "essential" information self-contained in the desidatamodel page, especially for the POTENTIAL HDU, which is a fairly short and straightforward.

Also: note that the extension name has changed: POTENTIAL_ASSIGNMENTS --> POTENTIAL

[sbailey]

+1 for the datamodel to be sufficient for interpreting the basic data format. Also see new ticket desihub/fiberassign#138 for discussion about exactly which columns should be included in this HDU to make it more user friendly.

[sbailey]

fiberassign now writes out a more user friendly potential assignments table that doesn't rely upon reading DocDB to know how to unpack it. Closing.