Explain how to handle quads and hexahedra 

Explain that we

* prefer users to split quads into triangles and
* require users to split hexahedra into tetrahedra

These are all well researched areas, we can surely link to papers in case someone needs to do this manually. https://github.com/precice/precice/issues/1324


https://precice.org/couple-your-code-defining-mesh-connectivity
