Performance: Speed up CGAL n-ary union

By utilizing CGAL::Nef_nary_union_3, n-ary union of many objects becomes significantly
faster (more than twice as fast in a test of 20 objects).

A further optimization potential lies in sorting the unioned objects based on proximity.
Merge branch 'unstable' into OskarLinde-nary_union_speedup
Refactored Nef_nary_union optimization