Irregular Bspline operator

[nmbader]

CubicSpline: Irregular Node Locations

This update extends the CubicSpline operator so the user can optionally specify non-uniform control-node locations along active dimensions.

• New capability: provide x1, x2, x3 as normalized node-location vectors for active dimensions 1 to 3.
• Input requirements: each x vector must lie in [0, 1], must not contain duplicate values, and must have the same length as the corresponding domain dimension.
• Internal handling: if a vector is not ordered, it is sorted internally; then the smallest value is forced to 0 and the largest value is forced to 1.
• Backward compatible: if locations are not provided, the operator uses the existing regular uniform spacing.
• Scope: works for 1D, 2D, and 3D active interpolation (with passive dimensions unchanged).
• Practical impact: enables spatially adaptive spline parameterization, with denser control where more detail is needed and coarser control elsewhere. In particular in the depth direction (x1).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.62%. Comparing base (f72248f) to head (5a954af).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   95.73%   95.62%   -0.11%     
==========================================
  Files          41       41              
  Lines        1148     1120      -28     
==========================================
- Hits         1099     1071      -28     
  Misses         49       49              

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