Control point deriv

[faymanns]

Implement partial derivatives w.r.t. control points for optimisation.

Adds methods to the spline class to compute the partial derivative with respect to the control points for the following functions:

• The spline it self and its derivatives
• The squared norm of the spline or one of its derivatives
• The reparametrisation energy

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.56%. Comparing base (20e67e5) to head (42daae3).

Files with missing lines	Patch %	Lines
src/splinebox/spline_curves.py	97.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   96.38%   96.56%   +0.18%     
==========================================
  Files           4        4              
  Lines        1271     1457     +186     
==========================================
+ Hits         1225     1407     +182     
- Misses         46       50       +4     

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