Add 3D circular beam bending validation: SOFA vs FreeFEM

[Fimache]

Context

Adds a new SOFA vs FreeFEM cross-validation case: a 3D circular
cross-section beam, cantilevered at one end, subjected to a transverse
(bending) load on the opposite face. Complements the existing 3D
validation cases (axial traction on circular section, torsion,
distributed load).

Contents

• beam3d_circular_tet.py: tetrahedral mesh generation (gmsh) for a
  circular beam, with 3 physical groups (fixed face, loaded face, lateral
  surface).
• sofa_beam3d_circle_bending.py: SOFA scene — beam clamped at x_min
  (FixedProjectiveConstraint), transverse load applied on the x_max face
  via consistent nodal traction forces (integrated over the loaded
  triangular faces).
• freefem_beam3d_circle_bending.edp: FreeFEM solve of the same
  problem (same geometry, same boundary conditions).
• comparaison_script3d_circle_bending.py: node matching by
  coordinates between the two solvers, RMS computed on the three
  displacement components (ux, uy, uz).
• params.json / params_beam3d_circle_bending.json: geometry,
  material, and load parameters.

Known limitation

Unlike the axial traction case (compared against the Saint-Venant
analytical solution), this bending case does not yet have an
analytical reference.