Fix: EDRReader fails when reading only one step

[BFedder]

Fixes #3999

Changes made in this Pull Request:

• the _read_next_step() method now checks if only one frame is present. If so, the StopIteration exception is not raised.

PR Checklist

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

[codecov]

Codecov Report

Base: 93.53% // Head: 93.53% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (74c0d6a) compared to base (d27a32a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4030   +/-   ##
========================================
  Coverage    93.53%   93.53%           
========================================
  Files          191      191           
  Lines        25062    25063    +1     
  Branches      3547     3548    +1     
========================================
+ Hits         23442    23443    +1     
  Misses        1099     1099           
  Partials       521      521           

Impacted Files	Coverage Δ
package/MDAnalysis/auxiliary/EDR.py	98.91% <100.00%> (+0.01%)	⬆️

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[hmacdope]

LGTM :)

[IAlibay]

If n_steps is defined a priori then you don't need to also define SINGLE_FRAME?

[BFedder]

Good point, cheers!

[IAlibay]

CI failure is fixed in develop, I'm happy with things as-is.