Add Gaia targets to the flux standad catalog.

The Flux standard star catalog v3.2 does not cover regions with low-Galactic latitudes or Dec<-30.  Those fields may be observed in the upcoming engineering run in March 2024 (e.g., open clusters).  We will try to use effective temperature estimates based on Gaia BP/RP spectra ("teff_gspphot") to select F-type stars.   The new flux standard star catalog will have four additional columns:

teff_gspphot (float)

teff_gspphot_lower (float)

teff_gspphot_upper (float)

fstar_gaia  (bool)