Add concordance correlation coefficient

[SkafteNicki]

What does this PR do?

Fixes #1194
Adds concordance correlation coefficient to regression package.
Depends on changes made in #1200, which needs to be merged first.

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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 #1201 (fd05b31) into master (8f7b0e7) will decrease coverage by 50%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1201     +/-   ##
========================================
- Coverage      86%     36%    -50%     
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  Files         191     193      +2     
  Lines       11345   11375     +30     
========================================
- Hits         9727    4058   -5669     
- Misses       1618    7317   +5699