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

- substitute_array_accesses_with_constants
- get_loop_hierarchy

- split groups have been ignored

- sweep updates pdf field in place, but does not load all the values
  before overwriting them
-> added a new transformation on AssignmentCollection that loads all
   read values first

-> by default sympy generates "fmin" C-function for sp.Min which
   is wrong for integer parameters

- was not used consistently before
- symbol names are expected to be valid C identifiers
- for complicated field names, the latex_name of field should be used

- small (length < 5) arrays with shape and stride information had to be
  memcpy'd to the GPU before every kernel call
- instead of passing the information as arrays, the single elements are
  passed
- leads to more function arguments, but simplifies GPU kernel calls

-> changes in all backends required

- put all buffer related stuff into separate functions
- should be functionally equivalent