made some changes to the integrator to raise warnings when the arrays that are wrapping have a reference count greater than 1, this shouldn't be the case if the wrapping is done for the first time when initialising the integrator. I also overload the set_func_gradient function to raise an error as this function is currently unimplemented because unnecessary in the way modified_fire has been written: the integrator always takes care of initialising the gradient. This function is in general unnecessary and implementing it would complicate the code unnecessarily, besides I don't see the need for it right now.