Add cascaded polygon union method to (multi)polygon seq

This wraps JTS' `CascadedPolygonUnion` as a method on a sequence of `Polygon` or `MultiPolygon`.

I wasn't too sure about the return type to use, so I used what the existing binary union was using - seemed appropriate. Tests mimic the binary union tests since both should have the same results.

I tried to find other operations looking at JTS/PostGIS docs that would operate on sequences of (multi)polygons, but couldn't find any.