Add number of rows to plot of entityset

[MarcoGorelli]

Pull Request Description

Add number of rows to graph of entityset, closes #722

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After creating the pull request: in order to pass the changelog_updated check you will need to update the "Future Release" section of docs/source/changelog.rst to include this pull request.

[rwedge]

Would you be willing to add a test case that checks the number of rows reported in the graph and checks the single row edge case?

[MarcoGorelli]

Sure - will do!

[codecov]

Codecov Report

Merging #727 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
+ Coverage   97.63%   97.64%   +<.01%     
==========================================
  Files         118      118              
  Lines       10205    10222      +17     
==========================================
+ Hits         9964     9981      +17     
  Misses        241      241

Impacted Files	Coverage Δ
featuretools/entityset/entityset.py	95.59% <100%> (+0.01%)	⬆️
...eaturetools/tests/entityset_tests/test_plotting.py	100% <100%> (ø)	⬆️

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[MarcoGorelli]

@rwedge How should graphviz output be tested? I've tried with a regular expression, please let me know if there's a better way / you'd like this done differently

[rwedge]

@MarcoGorelli the tests look good, thanks! Could you add yourself the list of contributors for the upcoming release in the changelog?

[rwedge]

Looks good! Thanks for the contribution!

[MarcoGorelli]

Happy to help, thank you for your tool!