Hexamesh boundary mesh

**Current status**: when computing the boundary mesh for a hexahedral mesh, it returns a triangle mesh instead of quad. Goal: create quad elements instead of triangles. Note: hexa mesh only makes sense for very easy domains, like a box domain.

**Main classes/functions**:

**Outline of the solution**
- [ ] Step 1
- [ ] ...

----
*Assigned to:*
*Wingman:*
