Provide a `copy_fun(f, deep=False)` helper function

[smarie]

This could rely on partial(f) if deep=False,

For deep=True we should rely on one of the recipes here:

• https://stackoverflow.com/questions/13503079/how-to-create-a-copy-of-a-python-function
• https://stackoverflow.com/questions/6527633/how-can-i-make-a-deepcopy-of-a-function-in-python/30714299#30714299