Forbid multiple boundary condition flags on the same cell in the flag field
It should not be allowed to set multiple boundary condition flags on the same cell in the flag field. This mostly happens on edges and corners of the domain boundary, even for the standard "geometry::initBoundaryHandling" function.
If two boundary condition flags are set to the same cell on the flag field, then the order of the boundary sweeps matters and leads to different results for different orders.