Fix interpolation and spline issues

[brisvag]

See commit message.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (afd2168) 42.94% compared to head (35e7df0) 42.68%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/blik/widgets/picking.py	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
- Coverage   42.94%   42.68%   -0.27%     
==========================================
  Files          10       10              
  Lines         489      492       +3     
==========================================
  Hits          210      210              
- Misses        279      282       +3     

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