Fixed default value for mdmc_average in Accuracy as per documentation

[burglarhobbit]

What does this PR do?

This PR fixes the default value in argument for mdmc_average from "global" to None as per the documentation of Accuracy. This is also supported by the default value of mdmc_average in the documentation and code for F1Score, Precision and Recall.

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• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1036 (e4a10f6) into master (4a28149) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1036   +/-   ##
======================================
- Coverage      95%     95%   -0%     
======================================
  Files         181     181           
  Lines        8004    8004           
======================================
- Hits         7582    7581    -1     
- Misses        422     423    +1     

[justusschock]

@burglarhobbit thanks for this PR. It seems that you need to adjust some of these failing tests before we can merge this PR :)