Proper motion support added to reference catalogs

DM-8828 was just merged, which adds proper motion support to reference catalogs as described by RFC-368. If proper motion is available then it will be applied to a specified epoch as the reference catalog is read. You may specify an epoch of None to prevent this correction.

Note that we do not yet support correcting parallax. Also the error propagation model is slightly naive (error in proper motion is used to increase the error in position, but covariances are ignored). However, we do plan to implement parallax support and more sophisticated error propagation. In anticipation of that we do support storing such data in reference catalogs.

Note that the schema of reference catalogs is documented in the doc string for lsst.meas.algorithms.LoadReferenceObjectsTask.

Regarding the PanSTARRs reference catalog ps1_pv3_3pi_20170110 that Paul Price generated: it does contain proper motions, but they are in a different format and so will be ignored.