PSyAD: add support for Calls

[arporter]

Currently PSyAD will raise an error if it encounters a routine Call. Ideally, it would also create the adjoint of the target of the Call (assuming it can be found). This is required in order to apply the TF-MAD algorithm to a kernel with an active stencil as this is best achieved by changing the body of a loop over stencil cells into a Call to a routine.