bug: use direct casadi bspline function for 1D & 2D cubic interp

[martinjrobins]

Description

Instead of using the casadi plugin system, use direct bspline function for 1D and 2D cubic interpolation.

I didn't implement 3D cubic interpolation because (a) scipy has no functions for bspline interpolation in 3D, and (b) we only test 3D linear interpolation so didn't think it was neccessary.

Fixes #4570

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Key checklist:

• [x] No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• [x] All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• [x] The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.27%. Comparing base (9a479cf) to head (d1977b6). Report is 1 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4572 +/- ## =========================================== - Coverage 99.29% 99.27% -0.02% =========================================== Files 300 300 Lines 22776 22795 +19 =========================================== + Hits 22615 22630 +15 - Misses 161 165 +4 ```

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[kratman]

@martinjrobins Thanks, I am testing it now before I review