fix visualization/draw ICP example and add warnings

[rxba]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes ICP registration crashes in visualization/draw #6887
• 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

A minor issue, but the ICP example in examples/python/visualization/draw.py requires the user to select point correspondences for a preliminary rough transformation before ICP registration, but users may not be aware of this. Currently, if no point correspondences are selected, the application crashes with an IndexError.

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

Here are the changes I've made:

• prevented the IndexError when running the ICP example without point correspondences and added a RuntimeWarning for the user to hightlight the need to select point correspondences.
• additionally prevented a KeyError when the "Source (yellow)" cloud is not contained in the first selection set and added a RuntimeWarning for this.
• refactored the existing do_ICP_one_set and do_ICP_two_set with new functions _prep_correspondences and _do_ICP to reduce duplicate code.

[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]

Thanks for this much needed bug fix! Looks good after adding one more check (see below).

[rxba]

Thank you for the feedback! I've added your suggested check, and also found two more things that needed checking to avoid a crash, which I also added:

• add check if only one set provided to two_set ICP
• add check if no Target (blue) points are provided to single set ICP
• add check if overall number of corresponding points is mismatching and report the mismatch so that the user can add to the existing sets

[ssheorey]

Thanks @rxba ! Looks good.