Re-enable `deepcopy` on `PolygonTree`s

[DanielVandH]

Haven't been able to get a proper MWE yet. Running the polygon_hierarchy test file just sometimes segfaults in the first code block. Going to disable deepcopy temporarily, but a better fix is needed. (deepcopy isn't really the best thing to do on triangulations anyway [and probably is broken somewhere anyway due to some aliasing between fields], so I don't even hate the idea of throwing if someone tries to deepcopy a Triangulation but too late.)