This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state

- all branches are added up

- counter-based philox RNG: counter/key is filled with cell coordinate
  and optional external parameters like block position and time step
- works on CPU and GPU - on CPU only for non-vectorized versions

- introduced more flexible "CustomCodeNode" that can inject
  backend-specific hand-written code

boundary conditions can specify how the index list should be built:
- list coordinates of domain or boundary cells (previous always inner)
- list all links or only the first link

- started with generalized free energy formulation

- update waLBerla package in order to make use of this function!

- removed warnings from generated code
- made generated code string deterministic, generating the same twice
  gives binary equally files now