Add cohort builder high level functions tests

[syroBx]

Cover cohort builder high level test for #37

• add_step
• get_state
• update_filter
• restore state
• plot_data
• stats
• remove_step

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.38%. Comparing base (086713f) to head (834a305).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #79      +/-   ##
==========================================
+ Coverage   66.41%   72.38%   +5.96%     
==========================================
  Files          11       11              
  Lines        1977     1977              
==========================================
+ Hits         1313     1431     +118     
+ Misses        664      546     -118     

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[galachad]

Looks good, please move the sakila rda file into test/data/sakila directory. We will be using it only for tests.

Please add the license info about sakila in readme file in test/data/sakila.