Adds new image metric - ERGAS

[vumichien]

What does this PR do?

Adds new Metric ERGAS #799

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #894 (9f09f12) into master (3653838) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #894   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         171    173    +2     
  Lines        7181   7237   +56     
=====================================
+ Hits         6830   6857   +27     
- Misses        351    380   +29     

[SkafteNicki]

LGTM :]

[Borda]

@vumichien mind check the:

E       b, c, h, w = preds.shape
E       ^
E   IndentationError: unexpected indent

[vumichien]

@SkafteNicki Thanks for helping me a lot