[shallowwater] Add viscous bottom shear stress Poiseuille flow test

What this MR does / why does DuMux need it:

This introduces a new friction law for the case of very thin shallow water flows (thin film) where viscous forces are dominating. Also adds a test case for poiseuille flow at low velocities and tiny water depth, where the walls are full-slip (tangential), no-flow (normal), and the bottom friction is that due to "parallel plate flow" (without the top plate) / plane Poiseuille flow.

Notes for the reviewer

TODOs

• Put viscous no slip in its own header -> Viscosity needs to come from volume variables
• Why is the viscosity in the viscous flux called turbulent viscosity? I guess it's the effective viscosity (including "eddy viscosity") Usually we split this into a regular viscosity part which comes from the fluid system and an effective part which would correspond to the turbulence model. This would also be helpful here.
• We could add viscosity like density in the volvars (without memory overhead and requiring that it's a constant for now -> no runtime overhead).

Checklist

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.