The wrapper should * inspect the given ´LocalOperator´and add all missing ´jacobian()´and ´jacobian_apply()` methods using numerical differentiation. * have a template parameter that causes **all** methods to be overridden by the numerical version (for testing purposes etc.). * provide a free-standing function `addNumericalJacobian(local_operator)` that returns a copy of the wrapper. * accept an lvalue reference to the wrapped operator and store a reference (or a pointer) to it. * Detect whether the wrapped operator implements the instantionary interface and expose that interface if required. This is part of #45.