Changed variable asel to sample_atom_group

[tushdemort]

Fixes #

Changes made in this Pull Request:
Changed the name of variable asel to sample_atom_group

PR Checklist

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

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📚 Documentation preview 📚: https://readthedocs-preview--4078.org.readthedocs.build/en/4078/

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (cf8d5ce) 93.56% compared to head (dec5f33) 93.56%.

❗ Current head dec5f33 differs from pull request most recent head 7634ffd. Consider uploading reports for the commit 7634ffd to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4078   +/-   ##
========================================
  Coverage    93.56%   93.56%           
========================================
  Files          191      191           
  Lines        25075    25075           
  Branches      4049     4049           
========================================
  Hits         23462    23462           
  Misses        1093     1093           
  Partials       520      520           

Impacted Files	Coverage Δ
package/MDAnalysis/analysis/rms.py	93.02% <ø> (ø)
package/MDAnalysis/coordinates/DCD.py	98.66% <100.00%> (ø)
package/MDAnalysis/coordinates/base.py	94.69% <100.00%> (ø)
package/MDAnalysis/coordinates/memory.py	96.22% <100.00%> (ø)

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[github-actions]

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[hmacdope]

@tushdemort this is currently being addressed in #4077, we will be unable to review this PR until that PR becomes stale. Also note that we want to deprecate asel which means issuing a warning and potentially accepting both kwargs

[hmacdope]

Block on duplicate