more robust when fiberassign in earlier expid

[sbailey]

This PR fixes #1272 where the "look in previous exposures for the fiberassign file" could end up grabbing the wrong fiberassign file. I originally was just going to turn off this look-back behavior since it isn't needed in normal ops, but there may be special test cases of setting up on a tile and then doing some manual exposures that could result in spectrograph exposures without a matching fiberassign file in the same EXPID, so the updated logic is:

• if the raw data has a TILEID keyword, insist that the fiberassign file matches that
• if looking to previous exposures without knowing the TILEID, check the data TARGTRA,TARGTDEC vs. fiberassign TILERA,TILEDEC for whatever fiberassign file is found and only accept it if they match

It also corrects a case where the coordinates file is missing fiber location information, by correctly flagging the fibers as FIBERSTATUS MISSINGPOSITION.

Example case of previous code finding and using an incorrect fiberassign file (now generates a RuntimeError because it can't find a good fiberassign file):

assemble_fibermap --overwrite -o blat.fits -n 20210114 -e 72405

Example case of correctly walking back to find a fiberassign file, albeit with missing info in the coordinates file resulting in FIBERSTATUS flags:

assemble_fibermap --overwrite -o blat.fits -n 20210220 -e 77103

This PR also introduces a utility function desispec.io.util.checkgzip(filename) that will look for a filename with or without the .gz extension; this is handy for cases like fiberassign where sometimes it is gzipped or not. We may also use this in the future with the pipeline if infrequently used files get post-facto gzipped and we want the code to easily find the right file either way.

[sbailey]

Note: the test coverage is better when run at NERSC, due to tests that require large/complex inputs and thus are not included in the github actions tests, but I have run unit tests of this branch directly at NERSC to include the assemble_fibermap tests.

[coveralls]

Coverage increased (+0.008%) to 25.682% when pulling f1d1166 on fibermap into 1273d08 on master.