pystencils_walberla: use type-safe getData instead of uncheckedFastGetData

Fixes #110 (closed).

The performance impact in release mode is negligible and type-checking is generally quite useful. Thanks to !258 (merged), one can even use vector-valued fields in pystencils, which was unsupported before, but did not produce an error.

Replaces pycodegen/pystencils_walberla!8 (closed).