Revamp ONSAS export to ParaView

[mvanzulli]

Closes #455
Closes #517
Closes #518

Summary

This MR adds a VTK export module to output ONSAS simulation data (displacements, strain, stress) for visualization in ParaView. Supports single time-step .vtu exports and .pvd time-series collections.

Key Features

• VTK Export: write_vtk generates .vtu files with nodal and element data.
• Time-Series Support: Generates .pvd collections for multi-step visualization.
• Modular Structure: Helpers for data extraction and formatting (write_node_data, write_cell_data, to_vtk).

Notes

• DISCLAIMER: Users are responsible for verifying exported tensors per element formulation.