Splines in PyMC

[OriolAbril]

Notebook url: https://github.com/pymc-devs/pymc-examples/blob/main/examples/splines/spline.ipynb
reviewer: jhrcook

[OriolAbril]

The notebook needs to be updated to follow the style guide. All notebooks must have a post directive in their first cell.

Ideally it would also use matplotlib directly instead of plotnine to reduce the dependencies on external libraries

[michaelosthege]

The notebook should also mention the SplineWrapper and explain the difference to the spline built in the notebook.
https://github.com/pymc-devs/pymc/blob/073e26b9c650ff9ae851ab9904cb3ad9c905a643/pymc/distributions/dist_math.py#L316-L319

[OriolAbril]

Updated by #274, moving it to v4 auto to signal there are still changes needed on the metadata. It is also being excluded from some of the pre-commit checks still, that needs to be fixed too before moving it to the "Done" column (by closing this issue)

See #274 (comment)

[kuvychko]

Working on this notebook, as part of PyMC Open Source session #2 with @PrinceAsiedu