kappa: Update bad pixel flag in regionmask output NDF
smurf: Allow map to be low pass filtered before forming mask

This can help prevent pixels oscillating in and out of the mask on
successive iterations, thus aiding smoother convergence.
smurf: Turn smf_sigmalcip into a generic function
smurf: Allow masks to be created using kappa:ffclean-like algorithm

See new config params ast|com|flt.zero_snr_ffclean. This can provide a
better way (compared to basic use of xxx.zero_snr_hipass) to restrict the
size of the source areas in the mask. This is because basic use of
xxx.zero_snr_hipass produced bowling round bright sources, which cases
sources within such bowls to be excluded from the mask.