Flatten solver hierarchy for perpendicular-flap

[uekerman]

As part of the tutorials restructuring we want to flatten the second (solver) hierarchy. For the flow-over-heated-plate case this is non-trivial.

We currently have: https://github.com/precice/tutorials/tree/restructure/perpendicular-flap

- OpenFOAM-CalculiX
- OpenFOAM-FEniCS
- OpenFOAM-deal.II
- SU2-CalculiX 

We want instead:

- images/
- fluid-openfoam
- fluid-su2
- fluid-nutils
- solid-calculix
- solid-dealii
- solid-fenics
- precice-config.xml
- clean.sh
- README.md

The Nutils case is available here (a deprecated version of the Python bindings is still used)

Challenge: one preCICE config needs to fit all. This means dimensions="2" everywhere and the same coupling data names (forces, no stresses).

Furthermore, we should get similar physical results for all combinations (the compressible SU2 here is an exception). This means, in particular, that we need physical parameters all solvers can deal with. Here: only small deformations (but still visible) and small Re (such that no upwind scheme is necessary, compare Sect. 4.2 here).

For further conventions please copy from the turek-hron-fsi3 structure: https://github.com/precice/tutorials/tree/restructure/turek-hron-fsi3

[uekerman]

done in #146