spatial/r3: Add Tetrahedron type

[soypat]

What are you trying to do?

Create tetrahedral meshes and manipulate them. This is common, but not limited to, finite element analisis, CFD.

type Tetrahedron [4]Vec

How does Gonum not allow you to achieve your goal?

Missing a tetrahedron type that complements r3.Triangle's functionality. It should be well integrated with r3.Triangle to ensure coherence i.e. Calling Normal on a face of the tetrahedron should point outwards.

Are you able to help contribute the feature?

Yes.