Allow repackaging of raw phosim calibration files, i.e.  bias, flats, darks.  The filename and header structure is the same as for a science image, but `IMGTYPE` rather than `SKYEXP` needs to be one of `BIAS`, `FLAT`, or `DARK`

Completion criteria:

pass the translation as a calibration product, so that 

astrometadata -p lsst.obs.lsst.translators translate MC_H_20000217_009999_R22_S00.fits

contains

observation_reason: phosim

observation_type: dark