Where possible, add a (deprecated, following the procedure in RFC-557 unless that RFC has been rejected by the time you get to this) backwards compatible API to PhotoCalib to enable it to act as a drop-in replacement for Calib.

Per discussion of 2018-12-19, the most important goal here is to provide conversion to/from magnitudes using the same API as Calib uses, but, in general, the aim of this ticket is to minimize avoidable disruption to “science users” when PhotoCalib replaces Calib, without imposting an unmanageable development load. (To meet the latter goal, I suggest timeboxing initial work on this to no more than a week or so before sending for review.)