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

- complicated pressure tensor derivation not required

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

- conceptionally not required: force needs to be in transformed coords
- for 3 phase model it makes no difference after bug was fixed in
  extract_gammas

- previous implementation did not work for van der Walls eos
- works now in V-P coordinates

- 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

- 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

- correction functions

- currently scalar multiplication (field-access * Matrix) does not work
  since sympy classifies field-accesses as iterable and tries matrix
  multiplication instead of scalar multiplication

- Added test for linear equation system and moved it to pystencils.utils

- new functions to easily extract stencil coefficients and visualize them
- Moved stencil functions from lbmpy to pystencils

- simpler function to init N phase model