Draft: Feature/pnm ff add interface pore body velocity

What this MR does / why does DuMux need it: For coupling pore network model to a free-flow domain, we couple the pore bodies to the free-flow grid cells. Until now, the velocity of the throat was used for calculating the slip velocity by a local Beaver-Joseph slip condition. However, the throat is not directly coupled to the free-flow grid cells, but the pore bodies. As also the cross-sectional area of the pore bodies comes indirectly into the coupling conditions (the faces of the coupled free-flow cells should have the same area as the cross-sectional area of the coupled pore body (at least in 2D)), the velocity in the pore body should be used for obtaining the slip velocity.

We (@Maziar, @anna_m_kostelecky) propose a simple scaling of the throat velocity by the fraction of throatCrossSectionalArea over poreBodyCrossSectionalArea.

Notes for the reviewer

TODO: insert text here

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