No change is different from negative change

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Ensemble agreement assigns members with exactly no change to the negative fraction #1690
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

In robustness_fractions

• Computes the fraction of negative change explicitly and returns it
• The agreement fraction is the maximum of the positive, negative or no change fractions.

Does this PR introduce a breaking change?

Yes, agree_frac has changed. However, it now better reflects its definition and usual expectations. And the case where "no change" is the largest group should not be very frequent, it usually happens with zero-bounded indicators.

Other information:

[aulemahal]

FYI @RondeauG

[coveralls]

coverage: 90.192% (+0.002%) from 90.19%
when pulling 0bb653e on fix-robustness-agreefrac
into 2d8726b on main.