Implements support of the box-facet coupling scheme for surface grids. Furthermore removes support for xi != 1.0 on surface grids for cell-centered schemes as this condition does not generalize for surface grids.
This is WIP because it depends on !1541 (merged), where the segment intersections of segments with polygons embedded in 3d space will be implemented. After this is done, it remains to do:
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Add test using box for three-domain test