Exposure catalogs

Exposure catalogs are used for diagnostics where tangential shear around exposure centers (and later, chip centers) is measured. Currently this file is generated from an OpSim catalog, but we need to figure out how to ingest it from the LSST stack. It may change form at that point.

The columns in an exposure catalog are:

Group	Name	Kind	Meaning
exposures	ap_corr_map_id	1D int64
exposures	bgmean	1D float64
exposures	bgvar	1D float64
exposures	colorterm1	1D float64
exposures	colorterm2	1D float64
exposures	colorterm3	1D float64
exposures	darktime	1D float64
exposures	date-avg	1D str
exposures	dectel	1D float64
exposures	exptime	1D float64
exposures	filter	1D str
exposures	fluxmag0	1D float64
exposures	fluxmag0err	1D float64
exposures	imgtype	1D str
exposures	magzero_nobj	1D int64
exposures	magzero_rms	1D float64
exposures	mjd-obs	1D float64
exposures	obsid	1D int64
exposures	psf_id	1D int64
exposures	ratel	1D float64
exposures	rotangle	1D float64
exposures	rottype	1D str
exposures	runnum	1D int64
exposures	skywcs_id	1D int64
exposures	testtype	1D str
exposures	timesys	1D str