This adds the following two metrics to the computeExposureSummaryStats
task:

maxDistToNearestPsf: the maximum distance (in pixels) of an unmasked pixel
                     to its nearest PSF star (i.e. star that was used in
                     the PSF modeling).

psfTraceRadiusDelta: the delta (in pixels) between the maximum and minimum
                     model PSF trace radius values evaluated on a grid of
                     points lying in the unmasked region of the image.  Set
                     to NaN if any of the model PSF trace values evaluate
                     to NaN.

These metrics may be used downstream as thresholded criteria for
inclusion in coaddition.

Namely, add a threshold on maxPsfTraceRadiusDelta.  The default value
was conditioned on the HSC-RC2 dataset.