Make AssignmentCollection iteratable

See merge request software/pystencils!17

- field access are now equal if their underlying fields compare as equal
  not only if the id of their fields are equal

- bugfix in vector-typing of piecewise functions
- cast_function is now a sympy atom - fixes problems with sympy > 1.1
- replace_inner_stride_with_one is a bit faster now

- if a field access is printed in compass or index notation seems to
  depend on the sympy version
- removed doctests that rely on the printed form

- Added test for linear equation system and moved it to pystencils.utils

- added isotropic version

- new functions to easily extract stencil coefficients and visualize them
- Moved stencil functions from lbmpy to pystencils

- constants are typed - no need any more for this parameter

- hash function was not correctly implemented
- sometimes this caused a bug where latex version was not correctly displayed

-> test_numerical_1D notebook

- defined ordering of headers
- lbm split kernels produced different ordering of assignments due to
  usage of a set