inner_or_boundary=False, the index vector contains the boundary cells, so the direction should point to the fluid cell.
inner_or_boundary=True, the index vector contains the fluid cells and the direction points to the boundary cell.
Technically, this constitutes an API change because it was possible to work around in your own
Boundary class. However, this problem was only introduced two months ago in !266 (merged), so we better fix this inconsistent behavior.