general: set pixels with anomalously-low variances to bad

Pixels with anomalously-low variances are given disproportionate
weight when mosaicking images and then show up as oddities in the
final map. This primitive is based on an algorithm outlined by David
Berry. First it forms a "weight" image by taking the reciprocal of the
variance, before calculating the statistics with successive clipping
at 5, 5 and 5 sigma. The maximum value in these statistics is used as
a threshold, converted back into a variance (by taking the reciprocal)
and the variance component of the original image is thresholded at
that value.

Creates a new image with suffix "_maskv".
general: add LAST parameter to proceed if Frame is last in Group

The OBSEND parameter is designed to work with live, online pipelines;
for offline use the Group lastmember method is more appropriate.
PICARD: use new primitive to mask anomalously-low variances

And switch file-deletion primitive to one that allows intermediate
files to be preserved until the end of processing.