Make a look-up-table (LUT) for FGCM calibration.
This task computes a look-up-table for the range in expected atmosphere
variation and variation in instrumental throughput (as tracked by the
transmission_filter products). By pre-computing linearized integrals,
the FGCM fit is orders of magnitude faster for stars with a broad range
of colors and observing bands, yielding precision at the 1-2 mmag level.
Computing a LUT requires running MODTRAN or with a pre-generated
atmosphere table packaged with fgcm.