peprpendicular-flap solid-openfoam case needs tuning

In #507, while comparing all the solver options we have, it became apparent that the solid-openfoam (solidDisplacementFoam) gives very different results than everything else. Still, it is a nice technical test that only requires OpenFOAM. The solid-solids4foam case is mostly fine already.

I have tried the following:

• Varying the mesh size: I get completely different results
• Varying the fvSolution/stressAnalysis/nCorrectors (suggestion by @philipcardiff): I get a bit different results, but not enough.

Some pictures that might help someone pick it up later on:

For now, I just give up, and document that this is an outlier at the moment.

One case use the plot-all-displacements.sh script to generate these pictures (see #507).

Start with the solid-openfoam case using solidDisplacementFoam and compare it with the solid-solids4foam case described in #507. Inspect fvSolution/stressAnalysis/nCorrectors and run plot-all-displacements.sh to reproduce the displacement and convergence plots. Done means identifying and correcting the discrepancy, or documenting the outlier and its reproducible conditions.