Fix the KD-Tree query return

[FanwangM]

The index of each neighbor of KD-Tree query results can be integer or array of integers. The original implementation works for the array of integers, but not including the integer case. This commit fixes the problem.

More explanations were provided in https://github.com/theochem/Selector/issues/236.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.91%. Comparing base (d931154) to head (4c5819d). Report is 13 commits behind head on main.

Additional details and impacted files

[](https://app.codecov.io/gh/theochem/Selector/pull/235?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem) ```diff @@ Coverage Diff @@ ## main #235 +/- ## ========================================== - Coverage 96.00% 95.91% -0.09% ========================================== Files 9 9 Lines 975 978 +3 ========================================== + Hits 936 938 +2 - Misses 39 40 +1 ``` | [Files](https://app.codecov.io/gh/theochem/Selector/pull/235?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem) | Coverage Δ | | |---|---|---| | [selector/methods/partition.py](https://app.codecov.io/gh/theochem/Selector/pull/235?src=pr&el=tree&filepath=selector%2Fmethods%2Fpartition.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theochem#diff-c2VsZWN0b3IvbWV0aG9kcy9wYXJ0aXRpb24ucHk=) | `98.03% <100.00%> (+0.02%)` | :arrow_up: |