pre-calculate stdstar model magnitudes

[sbailey]

When running stdstars super-fast on GPUs, @dmargala noted that re-calculating standard star template magnitudes every time is a significant contributor to the total stdstar runtime. These could be pre-calculated using standard filters needed by DESI (Gaia plus PHOTSYS=N/S for g,r,z) and saved in the $DESI_BASIS_TEMPLATES/stdstar_templates*.fits files themselves instead of calling desispec.scripts.stdstar.get_magnitude in desispec.scripts.stdstar.pyL521 . Note that there are other later calls to get_magnitude on the extincted flux of the selected model; this would still need to be custom.

I think this is independent of the recent work on dust, E(B-V), and Vega vs. AB but it touches similar code, so let's wait for PR #1246 and desihub/desiutil#169 before adding this optimization.

[julienguy]

Note PR desihub/desiutil#171 (newer version 169) has been merged.