Import psfex to finalize_characterization

The kernel and stamp sizes for PSF modelling of LATISS data are
required to be quite large, and piff will likely not scale well,
so we need the option to retarget to psfex to avoid long runtimes.

python/lsst/pipe/tasks/finalizeCharacterization.py

@@ -35,6 +35,7 @@
 import lsst.daf.base as dafBase
 import lsst.afw.table as afwTable
 import lsst.meas.algorithms as measAlg
+import lsst.meas.extensions.psfex.psfexPsfDeterminer  # noqa: F401
 import lsst.meas.extensions.piff.piffPsfDeterminer  # noqa: F401
 from lsst.meas.algorithms import MeasureApCorrTask
 from lsst.meas.base import SingleFrameMeasurementTask, ApplyApCorrTask