add sad ssd mae mse rmse

[johnnychen94]

This is part of the porting work https://github.com/JuliaImages/ImageDistances.jl/issues/17

• sad --> sad SumAbsoluteDifference
• ssd --> ssd SumSquaredDifference
• (deprecation) sadn --> mae MeanAbsoluteDifference
• (deprecation) ssdn --> mse MeanSquaredDifference
• new: rmse RootMeanSquaredDifference

There are two things I definitely need feedbacks:

• @timholy will the name deprecations sadn and ssdn good to you?
• SumAbsoluteDifference is equivalent to Cityblock, SumSquaredDifference is equivalent to SqEucleadean. Should I deprecate each one of them? (if yes, I prefer to deprecate Cityblock and SqEucleadean)

Note on git diff: The previous metrics.jl is renamed to metrics_distances.jl, to indicates that they are metrics exported by Distances.jl

[codecov[bot]]

Codecov Report

Merging #30 into master will increase coverage by 3.39%. The diff coverage is 80.76%.

@@            Coverage Diff            @@
##           master     #30      +/-   ##
=========================================
+ Coverage   65.71%   69.1%   +3.39%     
=========================================
  Files           5       6       +1     
  Lines         105     123      +18     
=========================================
+ Hits           69      85      +16     
- Misses         36      38       +2

Impacted Files	Coverage Δ
src/ImageDistances.jl	75% <ø> (ø)	:arrow_up:
src/metrics_distances.jl	62.5% <62.5%> (ø)
src/metrics.jl	88.88% <88.88%> (+26.38%)	:arrow_up:

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[johnnychen94]

merge since there's no feedback.

I'll revisit the codebase before 0.3 release