Improve the speed of read_spectra function

[biprateep]

Here are some of the suggestions, courtesy of Raga Pucha and John Moustakas:

• Use the read-only directory: '/dvs_ro/cfs/cdirs/desi/spectro/redux/{specprod}/healpix' instead of /global/cfs/...
• You can do the same thing for fastspecfit files as well: /dvs_ro/cfs/cdirs/desi/spectro/fastspecfit/...
• In the latest DESI kernel, they have improved the read_spectra() function. You can use "skip_hdus" keyword and it will run faster. Also, you can give a bunch of targets together for a given "SURVEY", "PROGRAM", "HEALPIX"
  spec = read_spectra(coadd_file, skip_hdus = ('EXP_FIBERMAP', 'SCORES', 'EXTRA_CATALOG')).select(targets = targetids)
• Tip from experience: "DO NOT SKIP MASK HDU". The masks are not applied when you are coadding the spectra across all cameras -- so, keep the MASK HDU for the correct spectra.
• When using multiprocessing, test for 128 or 64 processes in this line: pool = Pool(processes = 128) - I have noticed that 256 usually slows things down.
• You can use array jobs instead of a single job -- another way to make things run fast and get nodes assigned easier.