Add flapping constraint to wing root

[Ccaccia73]

I have created a new branch named flap-wing which defines a flapping law for the wing root.
Modifications impact:

• 10_utils folder: where I put a python script to generate the flapping law
• 05_FSI folder: the preCICE configuratio and the Solid part of the solution changes a bit in the model definition

If we find it interesting we can merge it into the main branch

[MakisH]

Sounds interesting, but I would keep it as an optional, advanced step.
In any case, feel free to open a pull request, so that we can discuss directly on the code changes.

[Ccaccia73]

Ok, I would propose to leave it as an advanced step in place of the other FSI_air case, which is a bit redundant.