adapt to change in LoadReferenceObjectsTask API

loadSkyCircle now takes an 'epoch' argument, intended for proper motion
corrections. However, our astrometry.net catalogs don't typically
support proper motion correction, and even when they do the format
of the data isn't certain, so no proper motion correction is done here.

python/lsst/meas/extensions/astrometryNet/loadAstrometryNetObjects.py

@@ -81,13 +81,21 @@ def __init__(self, config=None, andConfig=None, **kwargs):
         # because astrometry may not be used, in which case it may not be properly configured
 
     @pipeBase.timeMethod
-    def loadSkyCircle(self, ctrCoord, radius, filterName=None):
+    def loadSkyCircle(self, ctrCoord, radius, filterName=None, epoch=None):
         """!Load reference objects that overlap a circular sky region
 
         @param[in] ctrCoord  center of search region (an afwGeom.Coord)
         @param[in] radius  radius of search region (an afwGeom.Angle)
         @param[in] filterName  name of filter, or None for the default filter;
             used for flux values in case we have flux limits (which are not yet implemented)
+        @param[in] epoch  Epoch for proper motion and parallax correction
+                    (an astropy.time.Time), or None
+
+        No proper motion correction is made, since our astrometry.net catalogs
+        typically don't support that, and even if they do they format is uncertain.
+        Users interested in proper motion corrections should use the
+        lsst.meas.algorithms.LoadIndexedReferenceObjectsTask or they will need to
+        subclass and define how the proper motion correction is to be done.
 
         @return an lsst.pipe.base.Struct containing:
         - refCat a catalog of reference objects with the