copy method for prep objects

[tpapp]

Please add copy method for prep objects if that is feasible.

Rationale: no need to invoke prepare_... methods again, which require (at least the type of) the original x which would then need to be kept, for no other reason.

Specifically, I want to use it in LogDensityProblemsAD.jl, our current implementation is not thread-safe since copy falls back to a no-op.

[gdalle]

That's a very good idea!