Fetch ndcurves if not found

[ManifoldFR]

Following loco-3d/ndcurves#140 and loco-3d/ndcurves#142 we can use ndcurves as FetchContent

[ManifoldFR]

This change is part of the following stack:

• Fetch ndcurves if not found #26 ◀
  • Change all instances of taking argument "const Eigen::VectorXd &" to "const ConstVectorRef&" #27
    • Add pixi support and CI #28

_{Change managed by git-spice.}

[ManifoldFR]

We can merge this @edantec.