Convert triangle mesh model to tmesh map

[ssheorey]

• ToString() for Material and string represtion in Python.
• MaterialRecord to Material conversion - allows converting triangle mesh models to a map of tmeshes, including materials. Limitation: Only one material per tmesh.
• Support for emissive color reading in FileASSIMP (model) and writing for tmesh (tio FIleASSIMP)

Test with:

flight_helmet = o3d.data.FlightHelmetModel()
model = o3d.io.read_triangle_model(flight_helmet.path)
mesh_dict = o3d.t.geometry.TriangleMesh.from_triangle_mesh_model(model)
print(mesh_dict)
o3d.visualization.draw(list({"name": name, "geometry": tmesh} for
    (name, tmesh) in mesh_dict.items()))

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Emissive color is only partially supported. This PR adds complete support.
Currently, there is no way to store materials read from a gltf file (through read_triangle_model()) with a tmesh.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Tested works by user. Merging without review.