Partitioned-Pipe tutorial with fixedFluxExtrapolatedPressure

[thesamriel]

As I have established in my master's thesis, I would recommend to use the fixedFluxExtrapolatedPressure boundary condition for pressure on the second half of the pipe.
This way we can achieve results that are identical to a monolithic pipe simulation.

However, this boundary condition can only be used with SIMPLE/PISO/PIMPLE solvers and therefore not with sonicLiquidFoam

Solutions

• We can keep participant fluid2-openfoam-sonicliquidfoam with fixedGradient pressure and the resulting offsets in the solution.
• We replace the sonicliquidfoam halves with e.g. pisoFoam or rhoPimpleFoam

[MakisH]

Most important is that we have one case (just incompressible) demonstrating these BC.
The sonicLiquidFoam was a rather arbitrary choice when we were looking for a compressible solver to demonstrate that the adapter technically works with compressible solvers. rhoPimpleFoam (or even just another tutorial with buoyantPimpleFoam) would also be good.
We can completely drop the sonicLiquidFoam case, we have not promised any long-term support for this.