Fix Interpolator return type being a generator

[djhoese]

Also clarify language around kx_/ky_ arguments which are flipped from typical geotiepoints convention. Normally x is columns (cross-track) and y is rows (along-track), but RectBivariateSpline was provided coordinates in the (y, x) order so the kx_ argument is rows/along-track and ky_ is columns/cross-track.

Written by Claude with a lot of feedback from me.

@pnuu I never remember if you're part of geotiepoints development. Let me know and feel free to remove yourself from the reviewers.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/main **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (ec581f8) to head (0a01996).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #132      +/-   ##
==========================================
+ Coverage   89.58%   89.74%   +0.16%     
==========================================
  Files          20       20              
  Lines        1517     1541      +24     
==========================================
+ Hits         1359     1383      +24     
  Misses        158      158              

Flag	Coverage Δ
unittests	89.74% <100.00%> (+0.16%)	⬆️

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[coveralls]

coverage: 88.861%. remained the same — djhoese:bugfix-interpolator-cleanup into pytroll:main

[djhoese]

@mraspaud Claude says you're basically the sole author of all of this stuff so I think that means you have to review it.

[pnuu]

I remember I've pair-programmed with Adam 10+ years ago on parallellizing something, but I think that's it. Looks like something Martin needs to check anyway, so I'll remove myself from reviewers.

[mraspaud]

LGTM
Does it mean we go from lazy to eager? I guess that doesn't really matter in practice, just curious.

[djhoese]

Correct, but the only use Claude found for this was in Satpy where it is immediately unpacked: a, b, c = interpolator.interpolate(...).