Implement `__copy__` and `__deepcopy__` model methods

[ricardoV94]

Description

A couple of users on discourse got some nasty bugs when trying to do copy.deepcopy(model). Whatever the default is doing, it ain't great.

We have a clone_model in model/fgraph that we can use for both copy and deepcopy!

It should work in all cases except for the known issue with GP variables https://github.com/pymc-devs/pymc/issues/6883