staggered boundary generation: direction index should point to fluid cell when...

When inner_or_boundary=False, the index vector contains the boundary cells, so the direction should point to the fluid cell. When 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.