leaderboard scores limited to provided decimal points.

@ mention of reviewers`

@Didayolo @bbearce`

A brief description of the purpose of the changes contained in this PR.

In leaderboard scores are rounded to precision decimal points. precision can be provided in the yaml file as below

leaderboards:
  - index: 0 
    title: Results
    key: main
    submission_rule: "Force_Last"
    columns:
      - index: 0
        title: average auc
        key: auc
        sorting: desc
        precision: 3
      - index: 1
        title: average bac
        key: bac
        sorting: desc
        precision: 4

Issues this PR resolves

Solving the long decimal number for scores in leaderboard

Screenshot of solved problem

A checklist for hand testing

• run makemigrations, migrate and collect-static for testing

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge