smurf: All source pixels to be accumulated within an SNR mask

Using an SNR mask can introduce instabilities in which pixels close to
the SNR limit swap repeatedly from inside to outside the mask, thus
preventing convergence. Setting the new (AST|COM|FLT).ZERO_ACCUM
parameter to a non-zero value prevents this by accumulating the mask from
one iteration to the next. In other words, if a pixel is flagged as
source, it remains flagged as source for all further iterations -
"Once a source pixel, always a source pixel".
smurf: Allow convergence to be specified by max change rather than mean change

Useful if you are using numiter = ast.skip + 1 (i.e. all "preparatory"
iterations and no "real" iterations).
smurf: Update quality in initial sky map

When makemap is used on multiple chunks of data, the quality array in the
output map is a logical OR of the maps for the individual chunks. This is
not very useful for skyloop, so now the SNR-based masked actually used is
written back out to the initial sky map when the initial sky is first accessed.