think about potential issues with non-coplanar subinterfaces in 3d

In the context of simplices we can assume the 4 vertices defining the subinterfaces to be coplanar, however, this is not the case for the subdivision of hypercubes. Some functions such as unitOuterNormal() might need some adjustment. With a more abstract interface (see #2) we could also split the attached subvolumes into simplices and circumvent the problem