[WIP] rework SuperPixel and SuperPixelImage with Array interface

[johnnychen94]

The reasoning behind this rework:

• a superpixel is a collection of pixels (values and indices)
  • values is an array of colorants
  • indices can be tuple of ranges, CartesianIndices, or even a tile
• lazily do getindex(::SuperPixel, inds...) for both simplicity and performance.

[codecov]

Codecov Report

Merging #6 (bed6a7e) into master (a663fef) will increase coverage by 6.32%.
The diff coverage is 82.69%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   64.44%   70.76%   +6.32%     
==========================================
  Files           3        4       +1     
  Lines          45       65      +20     
==========================================
+ Hits           29       46      +17     
- Misses         16       19       +3     

Impacted Files	Coverage Δ
src/SuperPixels.jl	100.00% <ø> (ø)
src/types.jl	46.15% <46.15%> (-19.07%)	⬇️
src/synthesize.jl	95.12% <94.87%> (-4.88%)	⬇️
src/algorithms.jl	0.00% <0.00%> (ø)

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