rename SSIM

[Borda]

What does this PR do?

Renaming structural_similarity_index_measure and StructuralSimilarityIndexMeasure
part of #729

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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 #747 (ebaa6ab) into master (b4ceaea) will increase coverage by 0%.
The diff coverage is 84%.

@@          Coverage Diff          @@
##           master   #747   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files         170    170           
  Lines        6751   6760    +9     
=====================================
+ Hits         6411   6422   +11     
+ Misses        340    338    -2