On polygonize add option for extracting contours with marching cubes (gdal_contour) from probability/distance mask

[munshkr]

Currently polygonize allows the user to specify a threshold for binarizing a probability mask before polygonizing it, so that the resulting vector file contains only polygons with probabilty equal or greater than specified threshold.

An alternative that could return better shaped polygons is to extract contours by using the marching cubes algorithm in gdal_contour

For example:

gdal_contour -p -inodata -b 1 -fl {thresh} {src_mask} {dst_shp}

where {thresh} is the isolevel (i.e. probability) to extract.