allow several folders for inflating ledgers

[araichoor]

This PR prepares for the soon-to-come addition of extra secondary targets files, as:
$DESI_TARGET/catalogs/{dr}/{dtver}/targets/{extradir}/secondary/{program}/{extradir}targets-{program}-secondary.fits (with extradir!={survey})
See related PR desihub/desitarget#764.
Catalogs with extradir=main2 are expected to be created soon, but this PR should handle any other name starting with main (e.g. main3).

In fba_launch_io.py, those files are only used to add flux columns to the secondary catalogs read from the ledgers (which will contain all secondary targets, from main/, main2/, etc).

New steps:
The fba_launch_io.get_desitarget_paths() function records the extra existing secondary catalogs, proceeding as follow:

• first look for folders named $DESI_TARGET/catalogs/{dr}/{dtver}/targets/{survey}*, other than $DESI_TARGET/catalogs/{dr}/{dtver}/targets/{survey};
• then, for each found extra folder {extradir} look for an extra catalog $DESI_TARGET/catalogs/{dr}/{dtver}/targets/{extradir}/secondary/{program}/{extradir}targets-{program}-secondary.fits;
• store each of those extra catalogs in a "scnd2", "scnd3", ... key (sorted by extradir names)

Then in fba_launch_io.create_mtl() for the secondary targets case, the "regular" secondary catalog, along with any recorded extra catalog, will be used to get the flux column information missing from the ledgers.

Lastly, fba_launch_io.update_fiberassign_header() records in the header any found SCND2, SCND3, ... extra catalog.

Example of use:
With test catalogs with main/ and main2/ secondary catalogs:

export DESI_TARGET=/global/cscratch1/sd/adamyers/desi/target
export DESI_SURVEYOPS=/global/cscratch1/sd/adamyers/test-new-mtl-adds
fba_launch --tileid 999999 --tilera 0 --tiledec 0 --survey main --program DARK --dtver 1.1.1 --outdir ./

This change is backward-compatible, for instance (currently there only is a main/ secondary catalog):

export DESI_TARGET=$DESI_ROOT/target
export DESI_SURVEYOPS=$DESI_ROOT/survey/ops/surveyops/trunk
fba_launch --tileid 999999 --tilera 0 --tiledec 0 --survey main --program DARK --dtver 1.1.1 --outdir ./

Technical python remark:
I came up with: targ = Table(fitsio.read(targdir, columns = columns + ["TARGETID"], rows=rows)), because:

• I want the fast reading of fitsio with only few columns and some specific rows;
• I want to stack those with astropy.table.vstack.
  I m happy to implement any more elegant suggestion.

[araichoor]

I ve slightly edited the PR first message, to make it more explicit.
@geordie666 : just to confirm: for the main survey, in the future, the expected prefix for added secondary should always be main*, i.e.:
$DESI_TARGET/catalogs/{dr}/{dtver}/targets/main*/secondary/{program}/main*targets-{program}-secondary.fits
right? (like main2, main3, main3a, etc, but it will start with main")

[geordie666]

Yes, that's correct. The file for added secondary targets will always start with main* (will always be main2, main3, main4...).

[araichoor]

ok, great, thanks for the prompt confirmation!