Generalize assertion helper for N dimensions

[swvanbuuren]

This pull request introduces support for 4D linear interpolation in the codebase, including the addition of a LinearInterp4D class, enhanced assertion helpers for N-dimensional interpolation, and comprehensive tests for 4D interpolation functionality. The changes improve code reusability, extend functionality to higher dimensions, and ensure robust testing.

New functionality for 4D interpolation:

• include/linear_interp.hpp: Added the LinearInterp4D class, which extends LinearInterpND to support 4-dimensional interpolation.

Enhanced assertion helpers:

• tests/assertion_helpers.hpp: Refactored interpolation assertion helpers into a reusable InterpNDAssertions class for N-dimensional interpolation, and added support for 4D assertions via Interp4DAssertions.

Comprehensive testing:

• tests/test_linear_interp.cpp: Added multiple tests for 4D interpolation, including tests for linear, direct template-based, and non-linear functions. These ensure correctness and validate interpolation behavior at grid points and midpoints.
• tests/test_linear_interp.cpp: Introduced VectorN4 type alias for 4D vectors to facilitate testing.

[Copilot]

Pull Request Overview

This PR adds support for 4D linear interpolation by introducing a new LinearInterp4D class, generalizes existing assertion helpers into a reusable N-dimensional framework, and provides comprehensive tests for 4D interpolation scenarios.

• Introduce LinearInterp4D extending LinearInterpND for four dimensions
• Refactor 1D–3D assertion helpers into InterpNDAssertions template and add a 4D wrapper
• Add tests in tests/test_linear_interp.cpp covering linear, direct template-based, and nonlinear 4D interpolation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/test_linear_interp.cpp	Added VectorN4 alias and multiple 4D interpolation tests, including linear and nonlinear cases
tests/assertion_helpers.hpp	Refactored per-dimension helpers into InterpNDAssertions<T, N, FuncValT> and added convenience wrappers for N=1–4
include/linear_interp.hpp	Added LinearInterp4D class with ctor delegating to LinearInterpND<T, 4>

Comments suppressed due to low confidence (3)

tests/assertion_helpers.hpp:21

• [nitpick] The InterpNDAssertions class lacks a top-level comment describing its purpose, template parameters, and usage. Adding a brief Doxygen or block comment would help future maintainers understand how and when to use this helper.

class InterpNDAssertions {

tests/assertion_helpers.hpp:35

• [nitpick] The dim_name helper only handles up to 4 dimensions (x, y, z, w). If InterpNDAssertions is ever used for N > 4, dim_name will produce unexpected characters. Consider generalizing or documenting this limitation.

    static constexpr char dim_name(size_t i) {

tests/test_linear_interp.cpp:366

• Tests currently cover grid points and midpoints for linear and quadratic functions. Consider adding edge-case tests (e.g., out-of-bounds coordinates, non-uniform grid spacing) to ensure error handling and robustness of LinearInterp4D::eval.

}