- 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