Read the fcr photoCalib directly

The fcr object is an empty Exposure, which now contains a PhotoCalib object,
persisted directly (instead of having Calib headers FLUXMAG0/FLUXMAG0ERR), so we
get the fcr_photoCalib, instead of reading the metadata. This should also maintain
backwards compatibility with old files.

Update the dataRef mock in test_fluxFitBoundedField to be able to return an
fcr_photoCalib (which is a valid composite object for a real butler).

python/lsst/meas/mosaic/updateExposure.py

@@ -100,7 +100,7 @@ def getFluxFitParams(dataRef):
         ffpHeader = dataRef.get("fcr_hsc_md", immediate=True)
     else:
         ffpHeader = dataRef.get("fcr_md", immediate=True)
-    photoCalib = afwImage.makePhotoCalibFromMetadata(ffpHeader)
+    photoCalib = dataRef.get("fcr_photoCalib")
     ffp = FluxFitParams(ffpHeader)
 
     wcs = getWcs(dataRef)

tests/test_fluxFitBoundedField.py

@@ -143,6 +143,7 @@ def setUp(self):
                 "%d/fcr-%07d-%03d.fits" % (self.tract, self.visit, ccd)
             )
             fcrMetadata = readMetadata(fcrFilename)
+            fcrPhotoCalib = lsst.afw.image.ExposureF(fcrFilename).getPhotoCalib()
             self.ffp[ccd] = lsst.meas.mosaic.FluxFitParams(fcrMetadata)
             wcsFilename = os.path.join(
                 DATA_DIR,
@@ -157,6 +158,7 @@ def setUp(self):
             camera[ccd] = MockDetector(MockOrientation(nQuarter))
             self.dataRefs[ccd] = MockDataRef(visit=self.visit, tract=self.tract, ccd=ccd)
             self.dataRefs[ccd].put(fcrMetadata, "fcr_md", )
+            self.dataRefs[ccd].put(fcrPhotoCalib, "fcr_photoCalib")
             self.dataRefs[ccd].put(self.wcs[ccd], "jointcal_wcs")
             self.dataRefs[ccd].put(calexpMetadata, "calexp_md")
             self.dataRefs[ccd].put(camera, "camera")