- bugfix: loop counter of vectorized loop now correctly stored as SIMD vector with entries i, i+1, i+2, ...
- basis for in-kernel boundary handling

- allow AST nodes in assignment collection to e.g. put RNG nodes in
  LB method

- turned on restrict keyword by default (makes large difference on GPUs)
- smarter block indexing: changing block size depending on domain size
  Example: previously there where (1,1,1) blocks when requested
  block size was (64, 1, 1) and domain size (1, 512, 512), now the
  block size is changed automatically to (1, 64, 1) in this case
- added __lauch_bounds__ to kernels to allow better optimizations from
  the CUDA compiler

see https://github.com/sympy/sympy/issues/16662

- replace_density_and_velocity simplification produced terms like
  0 * omega, because sympy's auto-eval is turned off
- sympys CSE routine can apparently only handle evaluated terms
- solution: evaluate multiplications with zero (i.e. replace them by 0)

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

- all branches are added up

- was not used consistently before
- symbol names are expected to be valid C identifiers
- for complicated field names, the latex_name of field should be used

- small (length < 5) arrays with shape and stride information had to be
  memcpy'd to the GPU before every kernel call
- instead of passing the information as arrays, the single elements are
  passed
- leads to more function arguments, but simplifies GPU kernel calls

-> changes in all backends required

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

- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

- assignment collection
- sympyextensions

- Previously sympy.Eq was used to represent assignments. However Eq
  represents equality not assignment. This means that sometimes sympy
  "simplified" an equation like a = a  to True,
-> replaced sp.Eq by pystencils.Assignment everywhere
- renamed EquationCollection to AssignmentCollection

- shortened the longest running tests, moved some configurations
  to longrun