Add 3D MMS validation: Linear

[Fimache]

3D manufactured solutions with hexahedra (Q1)

• Linear MMS
• Neumann tractions integrated with 2‑point Gauss quadrature.
• Produces 1D and 2D displacement cut‑plots.

[th-skam]

Code works, tested on my side. Thanks to the ready plots, we can verify future changes.

I suggested some changes to re-use SOFA components instead of writing new functionality. Once done, we can move on to the next PR.

[th-skam]

It seems like the file in mms_validation was refactored into 3 separate ones (which is good). Is there a reason to keep both mms_validation and mms_structured?

Take a look at #20. I made some suggestions for your code.