```python
    def to_gpu(gpu_version, cpu_version):
        gpu_version = gpu_version.boundary_object_to_index_list
        cpu_version = cpu_version.boundary_object_to_index_list

>       if isinstance(self.data_handling, ParallelDataHandling):
E       TypeError: isinstance() arg 2 must be a type or tuple of types

../pystencils/pystencils/boundaries/boundaryhandling.py:102: TypeError
```

None is not a type! ParallelDataHandling is None if `waLBerla` is not
installed

this to your children or students)

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

- 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

- new time loop caches all kernel functions with their argument dict
  -> inner loop just calls C functions with cached kwargs

- not generic enough - does not work if there a multiple blocks

- flags can be force when setting boundaries, this is helpful for:
- setting up quantities (e.g. relaxation rate..) dependent on current
  flag configuration
- bit operation fix when pickling: function has to have the same name
  as the python object

- boundaries with same name have not been displayed correctly

- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

- Previously sympy.Eq was used to represent assignments. However Eq
  represents equality not assignment. This means that sometimes sympy
  "simplified" an equation like a = a  to True,
-> replaced sp.Eq by pystencils.Assignment everywhere
- renamed EquationCollection to AssignmentCollection

- in-kernel Neumann boundaries
- flag-interface for boundary handling makes one flag field multiple
  boundary handlings possible
- generator: support for bitwise logical operators

- splitted existing LBM boundary handling into two parts:
    -> generic part , that is used for FD as well and moved it to pystencils
    -> LBM specific part - remained in lbmpy

- bugfixes