Change coupling to:
pm receives pressure and communicates velocity

WIP

- Write routines to extrapolate pressure.
- Solvers use data from preCICE now
- Some hacks to make it work for now

- main_ff and main_pm will update preciceDataIDs of the problem after quantities are announced.
- PreciceAdapter uses its own indexing (always starting at zero) instead of preCICE indexing. This allows for easier data access
- member `dataVectors_` is increased in size as soon as a new quantity is added to the adapter

[ff-pm] mild integration of precice for ff-pm testcase; Need to integrate/reconstruct boundary values properly

added free flow + porous media flow coupling case from tutorial; The case should be at the state 'Task B: Include slip-condition'; Can we verify the case and split it into two parts?