GeometrySet design

[mhugo]

"Geometry sets" are used to compute boolean operations on geometries. Geometries are first decomposed into "primitives", that CGAL knows, then the computation takes place and then the resulted geometry set is "recomposed" to a geometry.
Some corner cases have implied strange design choices. For instance, polygon in 3D are decomposed by triangulation into 3D triangles. But these 3D triangles are all on the same plane. This information is useful for recomposition. There is then a flag associated to geometry primitives (IS_PLANAR). There should be better way to carry this information ...

[vmora]

Maybe introduce a primitive type FlatTin wich is a vector of Triangle_3 that come from the same polygon.