Update PSF modelling configs appropriate for LATSS

Namely:
(a) set the characterizeImage kernelSize and stampSize to 71
(b) change finalize to use a kernelSize and stampSize of 71
(c) change finalize PSF model from piff to psfex

Note that (c) is required (for now) because piff doesn't scale well
with increasing stampSize.

config/latiss/characterizeImage.py

@@ -15,6 +15,9 @@
 
 # Reduce psfex spatialOrder to 1, this helps ensure success with low numbers of psf candidates.
 config.measurePsf.psfDeterminer["psfex"].spatialOrder = 1
+# Set the default kernel and stamp sizes for PSF modelling appropriate for LATISS
+config.measurePsf.makePsfCandidates.kernelSize = 71
+config.measurePsf.psfDeterminer["psfex"].stampSize = 71
 
 config.installSimplePsf.width = 21
 config.installSimplePsf.fwhm = 2.355*2  # LATISS plate scale is 2x LSST nominal

config/latiss/finalize_characterization.py

@@ -1,2 +1,8 @@
-# Reduce piff spatialOrder to 1, this helps ensure success with low numbers of psf candidates.
-config.psf_determiner['piff'].spatialOrder = 1
+# Change the PSF determiner to psfex from piff default as the latter will not
+# scale well to the large kernel/stamp sizes for PSF modelling for LATISS data.
+config.psf_determiner.name = "psfex"
+# Reduce spatialOrder to 1, this helps ensure success with low numbers of psf candidates.
+config.psf_determiner["psfex"].spatialOrder = 1
+# Set the default kernel and stamp sizes for PSF modelling appropriate for LATISS.
+config.psf_determiner["psfex"].stampSize = 71
+config.make_psf_candidates.kernelSize = 71