Generated code uses uncheckedFastGetData

Using walberla::field::Field::uncheckedFastGetData in the template instead of walberla::field::Field::getData is dangerous. I kind of doubt it's so much faster that it would be worth forgoing the type checking. It's easy enough to accidentally switch around BlockDataID arguments to a sweep and you only notice because your results are wrong.

It seems like this is a workaround to permit retrieving e.g. a walberla::field::GhostLayerField<walberla::math::Matrix3<double>, 3ul> as a walberla::field::GhostLayerField<double, 27ul>, but there should be ways to do that in a type-checked manner. This workaround probably only works for layout zyxf anyway and would silently fail for fzyx.