Around keyword can use KDTree for periodic boundary conditions

[ayushsuhane]

Around keyword only used _apply_distmat if periodic keyword was active. Both brute force and kdtree routines can be used now for PBC and non-PBC calculations

Fixes partly #974

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[richardjgowers]

Can you rebase, the failures are unrelated

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@6b74bac). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             develop    #2022   +/-   ##
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  Coverage           ?   88.45%           
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  Files              ?      143           
  Lines              ?    17225           
  Branches           ?     2637           
==========================================
  Hits               ?    15237           
  Misses             ?     1389           
  Partials           ?      599

Impacted Files	Coverage Δ
package/MDAnalysis/core/selection.py	99.36% <100%> (ø)

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