import SegmentationMaskCollection in main starfish

[ambrosejcarr]

Adds an import statement so that SegmentationMaskCollection is findable in the root starfish module, like our other objects.

[codecov-io]

Codecov Report

Merging #1527 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1527      +/-   ##
==========================================
+ Coverage    87.1%   87.11%   +<.01%     
==========================================
  Files         139      139              
  Lines        4988     4989       +1     
==========================================
+ Hits         4345     4346       +1     
  Misses        643      643

Impacted Files	Coverage Δ
starfish/__init__.py	100% <100%> (ø)	⬆️

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