PICARD: don't create temporary Frame if there's only 1 file
PICARD: add MASK_LOWVAR parameter

This parameter determines if pixels with anomalously-low variances
should be masked out, usually before coadding/mosaicking.
PICARD: document MASK_LOWVAR and remove variance masking

The variance masking is now done within the mosaicking primitive to
allow control to be dictated there, rather than having a primitive
that is only used if a recipe parameter is set.
PICARD: mask out low variances here, update POD

Previously the low-variance masking was a step in the mosaicking
recipe. However, since it is only a stopgap to deal with maps created
before the new "hitslimit" config parameter was added, and as such
should only ever be optional, it is probably better to control it from
within the current primitive, rather than have it as a standalone step
and only executed if a recipe parameter is set.