Expose rotation submodule

[robjmcgibbon]

Rotations had to be imported explicitly, which I don't think should be required.

In [1]: import swiftsimio as sw
sw.
In [2]: sw.visualisation.rotation
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[2], line 1
----> 1 sw.visualisation.rotation

AttributeError: module 'swiftsimio.visualisation' has no attribute 'rotation'

In [3]: import swiftsimio.visualisation.rotation

[kyleaoman]

Fair enough. On the other hand, I've just had a look and this function is not used internally in swiftsimio itself anywhere. It is used:

• In the tests for rotated visualisations.
• In the documentation explaining how to make rotated visualisations.

The purpose is just to get the rotation matrix that aligns a vector with one of the coordinate axes. For that there's already scipy.spatial.transform.Rotation.align_vectors... I wonder if we could just drop this function, or make it a thin wrapper around that... anyway, for what it is this PR seems fine. Whenever you're ready just flag for review.

[robjmcgibbon]

Thanks. I just had someone yesterday coming to ask me why it wasn't working, so I don't think we should get rid of it since some people must be relying on it.