Allow vector assignments

Closes #17

See merge request !133

let show_code display/print code

Closes #6

See merge request !132

Add typeannotations and docstring for KernelWrapper

See merge request !136

- `get_code` will keep the previous behavior of `show_code`
- `get_code_str` will be equivalent to previous `str(show_code(...))`

Fix bug related to ParallelDataHandling

See merge request !130

Fix FiniteDifferenceStencilDerivation.rotate_weights_and_apply

See merge request !128

Print actual block contents in Conditional.{__repr__,__str__}

See merge request !131

```python
    def to_gpu(gpu_version, cpu_version):
        gpu_version = gpu_version.boundary_object_to_index_list
        cpu_version = cpu_version.boundary_object_to_index_list

>       if isinstance(self.data_handling, ParallelDataHandling):
E       TypeError: isinstance() arg 2 must be a type or tuple of types

../pystencils/pystencils/boundaries/boundaryhandling.py:102: TypeError
```

None is not a type! ParallelDataHandling is None if `waLBerla` is not
installed

Recent Numpy version do not support conversion from arbitrary iterables