Make the coupling stronger in perpendicular flap case

[Fujikawas]

With this change the coupling becomes stronger for the perpendicular flap case. It will require implicit coupling AND acceleration methods to converge. With $\rho=42$, it requires only implicit coupling, no acceleration is necessary. With current $\rho=3000$, both explicit and implicit coupling could lead to convergence. With $\rho=1$, the result from fluid-openfoam-solid-dealii looks like: With $\rho=42$, the result from fluid-openfoam-solid-dealii looks like:

Checklist:

• [ ] I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• [ ] I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[uekerman]

Worth considering on top: We could make the fluid inflow time-dependent. Then, also the $\rho=3000$ scenario could look interesting again.

[MakisH]

@Fujikawas please add in a comment the new figures you got in the context of https://github.com/MakisH/ofw19-training

[MakisH]

This has high educational value, as it demonstrates a case in which implicit coupling is important. To not trigger a cascade of side changes that require a lot of effort, let's convert this to a documentation note, encouraging the user to try a different density and observe the effect. (@Fujikawas)