Improve epoch_result_store code quality

[carmocca]

What does this PR do?

Fixes many code smells I found while debugging #4863
No logic changes.
I suggest reviewing the first commit separately. The other commit is just black changes.

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[justusschock]

I really like it. So far I only have one minor question regarding performance.

Probably @tchaton is the best one to review this!

[tchaton]

Awesome cleanup ! Thx.

[codecov]

Codecov Report

Merging #4875 (9c52318) into master (22bd742) will increase coverage by 0%.
The diff coverage is 94%.

@@          Coverage Diff           @@
##           master   #4875   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         129     129           
  Lines        9432    9359   -73     
======================================
- Hits         8739    8677   -62     
+ Misses        693     682   -11