- 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