Remove global search flag

[tommy-waltmann]

Description

This PR removes the global_search flag in EnvironmentCluster, in favor of having users just give a neighborlist where every particle is a neighbor of every other particle.

Motivation and Context

This PR makes the API more intuitive and less confusing.

Resolves: #984

How Has This Been Tested?

I have converted a previous test that used the global_search=True option.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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:

• I have read the CONTRIBUTING document.
• 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.

[vyasr]

@tommy-waltmann I didn't remember to mention this in earlier PRs, but one thing that might be helpful to maintain continuously on the next branch is some sort of migration guide. We can expand on it when it's actually time for the 3.0 release, but it would be helpful during each PR to maintain some minimal notes for how users should adapt to the change. In the case of removing global_search, it can be emulated by constructing a NeighborList of all pairs of particles as neighbors.

[tommy-waltmann]

@tommy-waltmann I didn't remember to mention this in earlier PRs, but one thing that might be helpful to maintain continuously on the next branch is some sort of migration guide. We can expand on it when it's actually time for the 3.0 release, but it would be helpful during each PR to maintain some minimal notes for how users should adapt to the change. In the case of removing global_search, it can be emulated by constructing a NeighborList of all pairs of particles as neighbors.

@vyasr That already exists, it's in doc/source/gettingstarted/migration.rst.

[vyasr]

Right sorry I forgot that we added that in #1013. In that case the action item would be to update migration.rst for this change as well (but I also see that this PR is a draft so no rush, I just did a batch review this weekend).

[vyasr]

Couple of minor comments but otherwise LGTM.

[vyasr]

Maybe worth showing how to construct all_pairs_nlist. Depends on #1051's new feature.

[tommy-waltmann]

Good suggestion. However, I'd like to wait until we have an official release out with the changes from #1051 on it before adding the utility method in the example.