- 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

- complicated pressure tensor derivation not required

- bug was caused by change in stride parameter passing

- relative application of stencil by passing in field accesses
- documentation + doctest

Make AssignmentCollection constructible/convertible from/to dict

See merge request software/pystencils!21

-> last concentration is automatically computed as 1-others
-> makes code more general