Add FreeFEM/SOFA validation project

[Fimache]

Summary

Adds the projet_validation module, containing validation cases comparing FreeFEM and SOFA simulation results across 1D, 2D, and 3D elasticity scenarios.

Contents

• cases/: validation test cases (bar_1d, case_2d, case_3d) including compression, distributed load, three-point bending, circular bending/traction, and torsion
• common/: shared utilities for environment setup, metrics, and plotting
• notebook_validation.ipynb: notebook consolidating validation results
• results/: output results from SOFA validation runs

Base branch

Branched from main-freefem-sofa.

[th-skam]

This has totally copied the files that already exist in our repository, no?
I see files that already exist in examples/Freefem/... validation