implement grouped polygonization

Can this work

obtain quadmesh, convert to edge table
group by pixel value and visit each group in turn
filter out all edges that occur twice with a group
create sf segment lines, and merge_lines them
st_polygonize that - this is still the killer, since the holes will be present but also filled

Given that last point we still get to the contourPolys problem that could work by sensible procesing of the st_polygonize output.

It can't be in this package but might be separated out for sf by including or cadging from quadmesh.

mdsumner / spex