[Feature] Find neighbours points within a given radius through Octree

[chunibyo-wly]

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

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

This feature implements the issue #4581 , which could return indices of the points used to construct the octree.

However, I have two concerns:

1. The first issue is that, the points are not stored in either the Octree class or the OctreeNode class, causing the new function needing the points to be passed in as input. This feels a bit weird and creates challenges for further development. What do you think about changing the behavior of the octree, for example, saving Octree to a JSON file, the point cloud could be saved separately, with a relative path referenced in the JSON file?
2. The second is the variable member origin in Octree means minimum corner of the Octree node. Would it cause misleading in the future?

[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.

[chunibyo-wly]

Hello, may I ask for any updates on this?