Replace most instances of starfish.image.Filter.Reduce with imagestack.reduce

[ttung]

Unless the reduction is happening multiple times, it's simpler to represent a stack reduction as imagestack.reduce

Test plan: travis thanks!
Depends on #1539

[codecov-io]

Codecov Report

Merging #1548 into master will increase coverage by 1.74%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1548      +/-   ##
=========================================
+ Coverage   88.25%     90%   +1.74%     
=========================================
  Files         152     223      +71     
  Lines        5245    8261    +3016     
=========================================
+ Hits         4629    7435    +2806     
- Misses        616     826     +210

Impacted Files	Coverage Δ
...core/spots/DetectSpots/test/test_synthetic_data.py	100% <0%> (ø)
starfish/core/experiment/test/test_experiment.py	100% <0%> (ø)
...y_table/test/test_intensity_table_serialization.py	100% <0%> (ø)
...tarfish/core/segmentation_mask/test/test_expand.py	100% <0%> (ø)
starfish/core/imagestack/test/test_export.py	100% <0%> (ø)
...intensity_table/test/test_synthetic_intensities.py	100% <0%> (ø)
...rfish/core/intensity_table/test/test_to_mermaid.py	100% <0%> (ø)
starfish/core/test/factories.py	88.97% <0%> (ø)
...sh/core/expression_matrix/test/test_concatenate.py	100% <0%> (ø)
starfish/core/test/test_synthetic_spot_creation.py	100% <0%> (ø)
... and 61 more

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