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
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\ No newline at end of file comment in "Changes" of this MR.