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

- all branches are added up

- 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

- 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

- updates in lbmpy benchmark
-> enables better numa placement

- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

Updated testing of kerncraft_coupling and some functions in
kerncraft_coupling in order to be compatible with the kerncraft version 5.1

- renaming because of clashes with types.py from other packages