- works also with ps.kernel decorator

- previously moved only SympyAssignments
- relevant for custom boundary nodes, which are now pulled before loops

- SSA form is checked
- loop independence condition is checked
- bug fix in Field.create_generic when using index_shape

- uses ast instead of text-based editing
- can handle Piecewise defined functions
- new decorator for simplified usage

- 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

- flags can be force when setting boundaries, this is helpful for:
- setting up quantities (e.g. relaxation rate..) dependent on current
  flag configuration
- bit operation fix when pickling: function has to have the same name
  as the python object

- boundaries with same name have not been displayed correctly

- new kernel creation function for kernels that write to staggered fields
- simplification of conditionals with integers uses ISL library now
  via islpy bindings

- removed warnings
- added flake8 as CI target

- moved Dockerfiles into separate directory
- tests for minimal setup on ubuntu & conda
- adapted setup scripts to correctly handle dependencies
- new conda environment files for users and developers