Add ST_DWithin

The following GDMS functions have been replaced by their H2GIS
equivalents:

        GDMS                    H2GIS
        ST_IsWithin             ST_Within  (h2spatial -- SFS)
        ST_IsWithinDistance     ST_DWithin (h2spatial-ext)

Note also that PostGIS contains the function ST_CoveredBy which seems to
do the same thing as ST_Within.