smurf: Allow map pixels with very few hits to be set bad

Map pixels with very few hits have unreliable variances. Any such pixels
that by chance have silly small variances will be given huge weights
(1/variance) when maps are coadded, resulting in speckles in the coadded
map. The new "hitslimit" parameter allows a lower limit to be specified
on the hits per pixel. Any pixels with hits below this limit are set bad.
The limit is specified as a faction of the mean hits per pixel, averaged
over the map excluding pixels that get zero hits.

The current default is zero (i.e. no extra pixels are rejected), but this
should probably be changed to some suitable small value at some point.
Doing so will result in maps being slightly smaller than before as edge
pixels (which is where the low-hits pixel usually occur) will be set bad.