disable nsgrid neighbor search

[orbeckst]

Expected behavior

neighbor search with capped_distance returns reliable and consistent results

Actual behavior

It doesn't.

• Wrong results from capped_distance() with "nsgrid" search and atom in the center of the box #2919
• Neighbor search with nsgrid.FastNS yields different pairs for swapped coords and search_coords #2229
• FastNS yields wrong results on simple test case #2345
• capped distance fails for very small (<0.1A) boxes #2670

See #2229 (comment) for detailed analysis of problems.

Course of action

Disable nsgrid-based neighbor search until we have a stable solution. Use PBC-pkdtree instead.

Impact

• slower distance-based searches and analysis (e.g. RDF)
• correct results in all cases

Code to reproduce the behavior

See linked issues.

Current version of MDAnalysis

• Which version are you using? (run python -c "import MDAnalysis as mda; print(mda.__version__)") 1.0.x, 2.0.0-dev
• Which version of Python (python -V)? any
• Which operating system? any

[IAlibay]

I'm working on a quick fix for this given the discussion in #2768

Do we want to solely remove nsgrid from 1.0.1 (i.e. only target master), or do we want to do so for develop too?

[lilyminium]

Closed in #3004.