Mean reciprocal rank (MRR) of leaderboard scores

[cmanzo]

@ mention of reviewers

@Didayolo

Purpose of the changes contained in this PR.

For a new competition that we will launch soon on CodaLab, we would like to use the mean reciprocal rank (MRR) as a summary metrics for the overall evaluation of a group of scores in the leaderboard. However, at the moment, Codalab only allows using the (weighted) average operation over the rank of scores.
This is mentioned in some comments on #2736 and #3449.

Issues this PR resolves

This PR includes the operation to calculate the MRR over leaderboard scores.

A checklist for hand testing

• create a competition bundle with a column of the leaderboard performing the MRR of some metrics. You can use the modified IRIS competition bundle.
• upload a few submissions to the competition.
• check that the MRR column provides the correct calculation and ranking, similar as in the screenshot below.
  

Any relevant files for testing

I have created a modified IRIS competition bundle that has, as leaderboard columns, both the MRR and the Avg of the original metrics (Prediction Score and Duration).

Checklist

• Code review by me and @JesusPinedaC
• Hand tested by me
• I'm proud of my work (code changes are very minor though!)
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge

[Didayolo]

I first merge this into codalab:mrr in order to trigger the automatic testing.

[codecov-commenter]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (927279f) 60.45% compared to head (f06e051) 60.38%.

Files	Patch %	Lines
codalab/apps/web/models.py	23.52%	13 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              mrr    #3451      +/-   ##
==========================================
- Coverage   60.45%   60.38%   -0.07%     
==========================================
  Files         173      173              
  Lines       11396    11411      +15     
==========================================
+ Hits         6889     6891       +2     
- Misses       4507     4520      +13     

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