Request: add bg_ version of split_union_polygon() and split_partitioned_union_polygon()

[jipclaassens]

Relevant operators to be implemented would be:

• [x] union_polygon
• [x] partitioned_union_polygon
• [x] split_polygon
• [x] split_union_polygon
• [x] split_partitioned_union_polygon
• [ ] polygon_connectivity

Also, make use of cgal.org,

• [ ] cgal_split_union_polygon
• [ ] cgal_union/join, cgal_intersect, cgal_xor, cgal_difference
• [ ] cgal_polygon_connectivity
• [ ] cgal_buffer, cgal_simplify, cgal_minkowski_sum

[MaartenHilferink]

See my recent comment of #756

[jipclaassens]

added [[polygon_connectivity]]