- splitted existing LBM boundary handling into two parts:
    -> generic part , that is used for FD as well and moved it to pystencils
    -> LBM specific part - remained in lbmpy

- bugfixes

- now an lbMethod or updateRule can be passed in directly
- made method/update rule/ast function independent i.e. no function
  uses the same parameters as on of the others

- 3 phase model works now with step sytem

- scaling interface width eta instead of surface tensions tau to correct
  interface profile & surface tensions

- added rudimentary test to prevent these kind of problems

- renaming because of clashes with types.py from other packages

- fieldLayout parameter default was not really used
- switched default field layout for sweeps that are generated for
  waLBerla, from a C to a Fortran layout (i.e. loops are ordered
  z,y,x from outer to inner, to match waLBerla fields)

- support for kernels that run over walberla ghost layers

- works for 2D models now as well
- moved lattice boltzmann code generation to lbmpy_walberla module

- square channel benchmark for evaluating spurious currents in Poseuille flow
- force model support in cumulant methods
- option to set initial velocity field for simple serial scenarios
- bugfix in simple force model

Michael Kuron authored Feb 13, 2017 and Martin Bauer committed Feb 13, 2017

This commit makes the Python code backwards compatible down to Python 2.7. Previously it would only run on Python 3.5 and up.

Problems fixed included:
- `time.perf_counter()` doesn't exist
- all classes need to be new-style
- `functools.lru_cache` doesn't exist
- only the last argument to a function call can be `*`-expanded
- the `nonlocal` keyword doesn't exist
- metaclasses are used with a different syntax
- `yield from` doesn't exist
- `tempdir.TemporaryDirectory` doesn't exist
- iterators need a `next()` method