Include position errors in reference catalogs

Our reference catalog schema (currently documented only in the docstring of LoadReferenceObjectsTask) does not have any facility to include the errors on the position coordinates of the reference catalog, and has the flux errors as optional fields. Jointcal needs these to properly weight the reference catalog in the fit.

All of the input catalogs that we use to produce reference catalogs should include position and flux errors, we just need to propagate those errors into our format of reference catalogs. I propose that we make, using the naming of RFC-333, coordErr (an IcrsCoord, to become a SpherePoint, containing xErr,yErr) and _fluxErr required fields in the refcat schema (the latter being required only if a given _flux field is included in the catalog).

This does not account for the position covariance xyErr: that would likely be negligible for many catalogs, but would be a required field (possibly coordErrCovariance, an afw.Angle^2) when it is known, and set to zero otherwise.

In addition to the above fields, reference catalogs will include the proper motions and errors defined in RFC-271, as well as parallax,parallaxErr (arcsecond), and radialVelocity,radialVelocityErr (km/s).

If any of these fields are unknown, they will be set to 0, but still included in the catalog.