Expose neighbors computed using local density via a neighbor list

[DomFijan]

Description

This pull request exposes neighbors computed using local density via a neighbor list. The local density computation currently averages the number of neighbors using the approach described in the documentation but does not expose the associated neighbor list. This enhancement computes and exposes the full weighted neighbor list based on the approach described in the local density documentation. The weighted neighbor list applies the same rules used to compute neighbors of particles, considering contributions based on distances as neighbor list weights.

Motivation and Context

@rayasare requested this be exposed.

How Has This Been Tested?

Tests and docs are TODO

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds or improves functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] I have updated the documentation (if relevant).
• [ ] I have added tests that cover my changes (if relevant).
• [ ] All new and existing tests passed.
• [ ] I have updated the credits.
• [ ] I have updated the Changelog.

[DomFijan]

I haven't checked how this impacts memory/performance. If I added a flag that skips computation of the neighbor list, would this be an ok fix, or do you just think we should just drop this?

[DomFijan]

In an offline discussion we have concluded that the same behavior can easily be achieved with current neighbor list infrastructure and that this code is not needed. Another issue was the fact that changes proposed impact the performance and memory for users using local density in its current form.