review, simplify, and finalize fiberassign output format

Related to dangling PR #254: review the fiberassign output formats, simplify/trim them, and finalize it on something we are happy to use for years to come. A non-exhaustive list of items to check:
* The lightweight `fba-*.fits` format was designed to minimize I/O and simplify debugging and studies with mocks, but it lacks the full information needed for actual observations, which are post-facto merged into the full `fiberassign-*.fits` format. The HDUs from `fba` are propagated forwards so that any QA scripts on the `fba` files also works on the `fiberassign` files, but this results in replicated information
* FAVAIL vs. POTENTIAL_ASSIGNMENTS
* FASSIGN vs. FIBERASSIGN
* How much targeting information should be kept for targets that were assigned?
* and should this be kept in the FIBERASSIGN HDU or a separate TARGETS HDU?
* How much targeting information should be kept for targets that were reachable but not assigned?
* previous default was everything, including for reachable SKY targets, but that involved an HDU with 80% default values due to most columns for Tractor-targets not applying to the much larger number of SKY targets...
* Do we need to maintain backwards compatibility with tiles that were already observed and have a non-ideal format?
* Find and review emails on the desi-data list (and possibly elsewhere) before re-inventing the wheel again

Finalize this before restarting observations in Fall 2020.

Review dangling PR #254 alongside the fba-*.fits and fiberassign-*.fits HDUs, then inspect relevant desi-data list emails before deciding scope. Done means an agreed, finalized output format that resolves duplicated HDUs, target-information retention, and compatibility for already observed tiles before observations restart.

Written by the indexing model from the issue text.