[WIP] Updates for Release 19.9

[weaverba137]

This PR updates the data model for release 19.9. This is not ready to merge yet, but please use this PR for comments about the data model for release 19.9.

[weaverba137]

Questions and Problems from 19.9

@sbailey please review!

DESISURVEY_OUTPUT

• No model file for exposures.fits, test-tiles.fits.

DESI_SPECTRO_DATA

• The datachallenge files are not a useful comparison to the model.
• See also comments on fibermap files below.

DESI_SPECTRO_REDUX

• Many files types are not generated by the datachallenge.
• I'm very worried that passing around huge fibermap/target tables is going to
  result in lots of inconsistencies between files as columns are added or
  removed.

DESI_SPECTRO_SIM

• No comparison possible for pix and simpix files.
• Only routine header keyword and column name cleanup are needed.
• Fibermap files no longer have HDU2, TARGETS.
• Simspec TRUTH table no longer has CONTAM_TARGET.
• Why are NUMOBS_INIT and PRIORITY_INIT int64, when NUMOBS_MORE is int32.
  And in any case, why are we planning for 2**32 or 2**64 exposures?
• There are a number of type changes in simspec, and some columns
  that used to have units no longer do. See the OBSCONDITIONS table especially.

DESI_TARGET

• Is there any way to more accurately represent the "official" pipeline layout?
  For example, is the position of fiberassign under DESI_TARGET reflective of
  actual operations?
• The fiberassign data model file is named tile-TILEID-FIELDNUM.rst but the files
  are named tile-TILEID.fits.
• What happened to the standards files?
• Why did skies.rst get named that? It's called sky.fits.
  Are these really meant to be the same? There are a lot of changes.
• mtl files have no units on any column. Some of those columns have to have units.
  In fact, units are missing from basically every DESI_TARGET file.
• mtl files have NUMOBS_INIT as int64, but it is int32 in other files.
• targets file do not contain PRIORITY?
• There are several minor differences among truth, sky, mtl that should be
  checked in detail.
• Why are ALLMASK_[GRZ] columns float32? The description is pretty clear
  that it should be an integer.
• SEED keyword in truth table looks dubious. Why not an integer with a comment?
• Layout of tile file FIBERASSIGN table is radically different from other
  files.
• Does every HDU in tile have to have the same keywords?

[sbailey]

Thanks for this review and update. There are a large number of open questions here, but I am going to merge this PR as progress and move the questions into a ticket for future work.