Refactor calculatedObjectives in NSGA2 optimizer

[XueSongTap]

related: #433

Refactor NSGA2 calculatedObjectives from std::vectorarma::Col to arma::Cube

Changes

• Replaced std::vector<arma::Col<ElemType>> with an arma::Cube<ElemType> for calculatedObjectives.

  • Layout: (numObjectives x 1 x populationSize)
  • Each slice corresponds to the objectives of one individual.

• Updated all related methods (EvaluateObjectives, FastNonDominatedSort, Dominates, CrowdingDistanceAssignment, etc.) to work with the cube representation.

Compatibility

• Public API is unchanged.
• Internally, calculatedObjectives is now a cube instead of a vector of columns.
• Existing tests build and run successfully after the refactor.

yxc@yxc-MS-7B89:~/code/2508/ensmallen/build$ ./ensmallen_tests
ensmallen version: 2.22.2 (E-Bike Excitement)
armadillo version: 12.6.7 (Cortisol Retox)
random seed: 0
===============================================================================
All tests passed (12616 assertions in 341 test cases)

[conradsnicta]

@rcurtin Should this go in before ensmallen 3 is released? Related issue: #433

[rcurtin]

Hey @XueSongTap, thank you for the contribution! I have some comments. If you can handle them we can get this merged.

[rcurtin]

I handled all the comments and merged in the master branch. If the tests pass I think this is good to go.

[rcurtin]

Thanks again @XueSongTap 👍

[github-actions]

Second approval provided automatically after 24 hours. 👍