- 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

- abstraction layer for selecting CUDA block and grid sizes
  - line based (was implemented before)
  - block based (new, more flexible)
-  new conditional (if/else) ast node, which is necessary for indexing schemes (guarding if)

- bugfix for CUDA kernels with variable field sizes
- extended tests for pystencils gpu kernels

- this extension is necessary for more generic boundary treatment
- cells can now be structs, i.e. contain different data types
- instead of having numeric index dimensions, one can use the index per cell to adress struct elements