Fix fairmode rounding issue

[thorbjoernl]

Change Summary

#1117 introduced an issue where floating point arithmetic or lack of rounding resultet in R stats outside of its permissible range [1, -1] when using fairmode=True. This PR fixes this issue, by rounding the calculated values in _prepare_stats.

Related issue number

Discussion in cams2_83 chat

Checklist

• Start with a draft-PR
• The PR title is a good summary of the changes
• PR is set to AeroTools and a tentative milestone
• Documentation reflects the changes where applicable
• Tests for the changes exist where applicable
• Tests pass locally
• Tests pass on CI
• At least 1 reviewer is selected
• Make PR ready to review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.59%. Comparing base (9e569d8) to head (72ec6b2).
Report is 1 commits behind head on main-dev.

Additional details and impacted files

@@            Coverage Diff            @@
##           main-dev    #1142   +/-   ##
=========================================
  Coverage     78.59%   78.59%           
=========================================
  Files           125      125           
  Lines         19964    19964           
=========================================
  Hits          15690    15690           
  Misses         4274     4274           

Flag	Coverage Δ
unittests	78.59% <100.00%> (ø)

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