Fix surrogate model copy operation

[uremes]

Summary:

GPyRegression class both overrides copy.copy with its own copy method and tries to use copy.copy inside the copy method so that both copy methods result in a recursion error. Removing the override resolves the error.

How to test:

Run the BOLFI notebook and try bolfi.target_model.copy() and/or import the copy module and try copy.copy(bolfi.target_model).

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