Sasmodels shape visualization

[wpotrzebowski]

Summary

This PR adds 3D shape visualization for sasmodels: models can declare support, build meshes from form-factor parameters, and callers can render figures for docs and reports.

Relation to SASBDB

This work supports SASBDB report functionality on SasView (SasView/sasview#3912)

Changes

• ModelInfo: add has_shape_visualization, read from each model module (sasmodels/modelinfo.py).
• Form-factor models: set has_shape_visualization and implement create_mesh where a 3D shape exists; other models set has_shape_visualization = False.
• explore/shape_visualizer.py: shape detection, 3D plots (optional cross-sections), and generate_shape_image for non-interactive PNG output (API for host apps).
• explore/sasmodels_shape_gui.py: small GUI to browse models and preview visualizations.
• explore/add_visualization_to_models.py: helper to batch visualization code into model files.

Dependencies

Visualization uses matplotlib and numpy; suitable for optional explore tooling and opt-in image generation in applications.

[pkienzle]

This is a lot of code to maintain. Can you instead create a 3d model representation for each shape and use other tools for meshing and cross sections?