Spline interpolation

Description

Adds spline-based interpolators

Checklist

Please confirm that this pull request has done the following:

• Followed up !52 (comment 1770055515). Decision: just write our interface to the lapack function we need. It is super simple and fine for now. If we want lots of functionality in future, use something like https://github.com/perazz/fortran-lapack (make issue to capture this)
• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/