bug: use casadi MX.interpn_linear function instead of plugin #3783

[martinjrobins]

Description

Partially fixes #3783

• use casadi MX.interpn_linear function instead of interpolant for linear interpolation
• cubic interpolation not changed as I can't find a raw function for this that avoids the casadi plugin system

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

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

Key checklist:

• 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)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• 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:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (fd7d1fd) to head (76e0cce).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4077   +/-   ##
========================================
  Coverage    99.58%   99.58%           
========================================
  Files          260      260           
  Lines        21358    21365    +7     
========================================
+ Hits         21270    21277    +7     
  Misses          88       88           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[brosaplanella]

Looks good, thanks! Can you add a CHANGELOG entry before merging?