add group counts for group Box

[khillion]

Changelogs

• Add option to display counts in legend for group plots

Definition of Done

• [x] New code is tested with unit tests
• [x] Documentation has been updated
• [x] Pull Request has been revised by a maintainer of the project

[codecov-io]

Codecov Report

Merging #69 (4a35cfb) into master (b0610bb) will decrease coverage by 0.01%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   56.81%   56.80%   -0.02%     
==========================================
  Files          58       58              
  Lines        2246     2250       +4     
==========================================
+ Hits         1276     1278       +2     
- Misses        970      972       +2     

Flag	Coverage Δ
unittests	56.80% <50.00%> (-0.02%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
moonstone/plot/graphs/base.py	80.00% <50.00%> (-2.36%)	:arrow_down:

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