ENH: atleast_nd delegation

[Enderdead]

This merge request introduces the atleast_nd delegate function (#100).

Unlike other delegate functions, atleast_nd does not exist for all major backends. Instead, the functions atleast_1d, atleast_2d, and atleast_3d are available.

In this implementation, I use getattr to delegate when ndim is between 1 and 3. I’ve also added tests with ndim=3 to cover the full range.

Open to feedback!