This MR adds a coupled stokes/darcy test for which an analytical reference solution exists, taking into account the Beavers-Joseph-Saffman slip conditon.
The test checks for convergence. Three different reference solutions can be chosen on run-time.
We perform the test for the second example of Shiue et al., 2018: "Convergence of the MAC Scheme for the Stokes/Darcy Coupling Problem"
The test comes with a python script that helps to check the mathematical consistency of the reference solution. It also provides the necessary source term. It can therefore help to add new analytical solutions.