Test interp nominal lon

[JoranAngevaare]

Fix test for _interp_nominal_lon

This PR is some leftover work from #296 (and #300)

• I've added a dummy test - which is a slimmed down version of the example issue in #295.
• Add type-hints to #296
• Fix #358

PR checks

• [x] Closes #295, #358
• [x] Tests added
• [x] Passes pre-commit run --all-files
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [x] New functions/methods are listed in api.rst