find (nested) scheme for octahedron

[nschloe]

One can split a tetrahedron into four tets and an octahedron.

An oct can be split further as well:

https://en.wikipedia.org/wiki/Tetrahedral-octahedral_honeycomb

When finding a scheme for the octahedron (with error estimation), one can build an adaptive integrator for 3D from tetrahedra.