Deepcopy doesn't work

[martin-t]

Attempting to make a deep copy and modify it

from copy import deepcopy

stl = admesh.Stl('hellskull.stl')
stl_copy = deepcopy(stl)
stl_copy.rotate_z(5)

results in admesh.AdmeshError: STL not opened.

[hroncok]

This is how it's done in the Ruby bindings. https://github.com/admesh/radmesh/blob/78ab1d369a12708f1a52d1abf6fe278dfebb370c/lib/radmesh/stl.rb#L705

In Python, we need to define the __copy__() and __deepcopy__() methods. The first will probably just share the C object pointer, the second will mimic the Ruby behavior.

Idea: Add a stl_deepcopy or stl_clone function to C ADMesh and use it both here and in the Ruby bindings.