ENH : Compute the force exerted by the flow on the piston

[emvalbuena]

This is an good post-processing variable whose difference to compute between the FOM and the ROM models.

[emvalbuena]

It's actually not, because at the piston I know the velocity (boundary condition), so if I directly plug it into the pressure expression I would get the piston force, but this is always going to be correctly computed.

This would be more interesting in an FSI problem, where the solution at the piston is unknown.

[emvalbuena]

This could be interesting to validate the problem.