specmask.BADFIBER

[julienguy]

This PR fixes a bug/feature in the fiberbitmask functions, where the specmask.BADFIBER bit was set in the frame.mask whenever a fibermask.BADAMP{B,R,Z} bit was set in the fibermap['FIBERSTATUS'], irrespectively of the frame camera.

As an example, with the change in this PR, the quasar spectra from SP1 in tile 1299 are recovered.
See https://data.desi.lbl.gov/desi/spectro/redux/jguy/tiles/pernight/1299/20210707/tile-qa-1299-20210707.png
compared to https://data.desi.lbl.gov/desi/spectro/redux/guadalupe/tiles/pernight/1299/20210707/tile-qa-1299-20210707.png

This PR fixes issue #1673

[coveralls]

Coverage increased (+0.004%) to 24.62% when pulling 1dbffa9 on specmask-badfiber into b6fbf0d on master.

[akremin]

You can ignore these comments if you wish, but it naively looks to me like you are ignoring amplifier bits on the actual cameras for which they apply, which I'd like more explanation on.

[julienguy]

(retesting code before committing after accounting for Anthony's comments)

[julienguy]

@akremin the code ready for second review.

I checked it's producing the expected results (good redshifts despite lack of r-camera data for some fibers)

cd /global/cfs/cdirs/desi/spectro/redux/jguy/tiles/pernight/1299/20210707
plot_spectra -i coadd-1-1299-20210707.fits --rebin 6 --redrock redrock-1-1299-20210707.fits -t 39628397629932704

[akremin]

The updates addressed my comments. I made a doc string update to clarify that multiple bands are acceptable.