Face orientation convention for structured meshes is wrong

A mesh face can have at most two adjacent cells, C1 and C2. By convention the face is oriented according to an outward facing normal with respect to C1.

However, for structured meshes this convention is not enforced. For example, in 2D:
- All ``I``-Faces normals are pointing to the ``(+I)`` direction
- All ``J``-Faces normals are pointing to the ``(-J)`` direction

This can be problematic for operations that rely on the convention.

This should be relatively straight-forward to fix, by carefully handling the left-most boundary.