include growdiff.json in MANIFEST

[PeterDSteinberg]

Fix MANIFEST.in to include growdiff.json

[codecov-io]

Codecov Report

Merging #1217 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1217   +/-   ##
=======================================
  Coverage   98.83%   98.83%           
=======================================
  Files          39       39           
  Lines        3002     3002           
=======================================
  Hits         2967     2967           
  Misses         35       35

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