Add rotation matrix from orthonormal base

[berendkleinhaneveld]

Maybe call it mat_from_axes?

[Korijn]

We could. It would be trivial (arr = np.eye(4); arr[:3, :3] = axes), but there's value in defining the convention (rows vs columns).

Any other special requirements? For example, automatic pre-normalization of the input axes, or forced orthogonality?

[berendkleinhaneveld]

Your examples sound good to me!