smurf: Prevent mapchange being dominated by bad edge pixels

Pixels round the edge of the map may contain very few samples, and so
their variances can be widely inaccurate. So exclude pixels with low hits
from the estimate of the normalised map change (which is the basis of the
convergence test). "low hits" is anything under 10% of the mean hits per
pixel across the map.
smurf: Correct pixel bounds for JSA tiles in the split HPX facet

The split facet uses RA=12H as the reference point, and so the pixel
bounds of tiles in this facet need to be corrected to refer to the
expected NDF origin at RA=0h.
smurf: Correct mapbounds fast-mode so that it can be used for JSA maps

The previous version assumed that north was upwards, which it isn't for
JSA. For instance, try

% makemap in=/star/share/smurf/s4a\* crota=45 \
          config=/star/share/smurf/dimmconfig.lis

The issue was that the extrema time slices were found by looking at the
bounding box in (ra,dec) offset, but if the pixel axes are not aligned
with (Ra,Dec), then that's not the right box to use. Now, the boresight
positions are transformed into output map pixel coords, and the bounding
box in pixel coords is used to find the extrema time slices.

In addition, there may be some time gain due to the fact that the new
scheme uses astTran2 to transform all time slice positions simultaneously,
rather than making a separate call to astTran2 for each time slice.