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

- 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

- 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