WIP: Feature/add turbulent diffusion to shallow water model

Adding (turbulent) diffusion to the shallow water model (part of freeflow):

For now using constant eddy-viscosity. Turbulence models will be added in separate issues. The implementation also contains 2 wall shear stress implementations:

1. no-slip implementation
2. law-of-the-wall implementation

One test has been added: Poiseuille flow test: for flow in a channel with rough side walls. This test has an analytical solution.

Which issue this MR fixes: #706 (closed)

Special notes for your reviewer: N/A