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

- 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

- when running multiple pystencils instances, sometimes errors happened
  because one process might have partially written a cached file, which
  is already read before writing was finished
-> switched to "atomic write" (only on linux yet) that uses os.rename
   which is guaranteed to be atomic

- test run again
- notebooks not yet

- first vectorization tests are running
- type system: use memoized getTypeOfExpression
- casts are done using sp.Function('cast')
- C backend adapted for vectorization support
- AST nodes can required optional headers

- pystencils can create now a non-compilable kernel that can be
  analyzed by kerncraft