Add 3D sphere mesh example with PyVista visualization

[tkoyama010]

Add 3D Sphere Mesh Example

This PR adds a new example demonstrating how to generate and visualize a 3D sphere mesh using Gmsh and PyVista.

Features

• Generation of a unit sphere mesh (radius = 1.0)
• 3D mesh using tetrahedral elements
• Mesh file output (.msh format)
• Visualization using PyVista:
  • Multiple viewpoints (top, front, side views)
  • Edge display
  • Coordinate axes display
  • Semi-transparent surface rendering

Technical Details

• Mesh quality optimization
• Surface mesh extraction
• Support for off-screen rendering
• Type-annotated code with docstrings
• Follows project's code style guidelines

Example Output

The example generates:

• A Gmsh mesh file (sphere.msh)
• Three visualization images:
  • sphere_xy.png (top view)
  • sphere_xz.png (front view)
  • sphere_yz.png (side view)

Dependencies

Uses standard project dependencies:

• Gmsh
• PyVista
• NumPy

Please attach the generated mesh images when they are available.