Divide a single polygon (with several outer-contour components) into
several polygons, a polygon for each outer contour, by modifying the
PID and SID indices.
dividePolys (polys)PolySet with one or more polygons, each with possibly several components/holes.
PolySet, possibly with the addition of an SID column if
it did not already exist. The function may also reorder columns such
that PID, SID, POS, X and Y appear
first, in that order.
Given the input PolySet, this function renumbers the PID
and SID indices so that each outer contour has a unique PID and
is followed by all of its holes, identifying them with SIDs
greater than one.