TileFetcher formats complete 496 fov MERFISH dataset

[ambrosejcarr]

• Update tilefetcher to format complete dataset instead of 1 fov

Testing strategy:

import starfish.data
exp = starfish.Experiment.from_json(https://d2nhj9g34unfro.cloudfront.net/browse/formatted/MERFISH/20190511/
exp  # do things, verify 496 fovs

[codecov-io]

Codecov Report

Merging #1341 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1341      +/-   ##
==========================================
+ Coverage   89.02%   89.04%   +0.01%     
==========================================
  Files         147      147              
  Lines        5295     5295              
==========================================
+ Hits         4714     4715       +1     
+ Misses        581      580       -1

Impacted Files	Coverage Δ
starfish/core/types/_constants.py	100% <0%> (+1.88%)	⬆️

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