Adds new image metric - d_lambda

[ankitaS11]

Please see the previous PR here which was accidentally closed (the repo got deleted...).

What does this PR do?

Adds New Image Metric - D_Lambda (Spectral Distortion Index)

Fixes a part of #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 #873 (70c4995) into master (97f2bf5) will increase coverage by 0%.
The diff coverage is 97%.

@@          Coverage Diff          @@
##           master   #873   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files         173    175    +2     
  Lines        7401   7468   +67     
=====================================
+ Hits         7029   7094   +65     
- Misses        372    374    +2     

[stancld]

Some minor comments, otherwise LGTM already from #855

:)

[Borda]

@SkafteNicki @justusschock pls ^^

[SkafteNicki]

Few commets, else LGTM

[ankitaS11]

Few commets, else LGTM

Hi @SkafteNicki, I have updated the branch, please take a look again whenever you have the time :)

[SkafteNicki]

final comments (just for consistency in naming)