Overhaul astrometric calibration logic #82
Description
The current astrometric calibration strategy is to check FITS files for the first with enough stars before attempting calibration with solve-field, and to continue until the astrometry of a FITS file has been solved to act as the reference calibration.
Due to clouds, inadvertent camera movement/flexure or intentional slewing, the pointing may change during an observing run and a renewed astrometric calibration may need to be computed. Logic should be implement to detect failure of the calibration from the reference, and solve-field needs to be run again. This can fall back to searching around the last best known position, and if this fails, revert to an unconstrained solve (except perhaps with fixed pixel scales).