QLF pre-check of what cameras exist

[sbailey]

Migrating some QLF items from desihub/desispec#708, originally about CHECKHDUS which is now deprecated:

From emails discussions, the conclusion was basically to drop CHECKHDUs, since ICS will be handling the equivalent alarm condition of missing data.

The consequence for QLF:

• don't check for CHECKHDUs anymore; it will be removed from a future version of QL output
• it should check the input raw data file desi*.fits.fz for which cameras exist, and spawn QL only for those cameras
• any cameras that are not in the input files should just be grayed out for the display
• QLF should cache the results of what it found for each exposure, so that it doesn't have to re-read the desi*.fits.fz file just to know what cameras should be gray vs. displayed every time the webpage is loaded.

Note: it will be possible that 1 or 2 cameras for a spectrograph will exist, even if the 3rd camera is broken, e.g. having b2 and z2 but not r2.