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

- helps when simulating a large amount of small scenarios

- periodic kernels not yet OpenMP parallel

- cache relied on uniqueness of  python id()
- id may be reused if object is freed
-> object must be held alive
-> kernel keeps all it arguments it was ever called with, alive (problematic in terms of memory consumption)

previously the value was taken from the cell next to the boundary cell (i.e. the inner cell)

-> smaller block

-> moved llvm and dot import in try-except block

- VC 15 (2017) does not set an environment variable any more
- fallback to filesystem search