Add "chain__", "iter__", and "draw__" columns to draws_pd

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Closes #676

Copyright and Licensing

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #677 (feed16c) into develop (1361f35) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #677      +/-   ##
===========================================
- Coverage    81.06%   80.99%   -0.07%     
===========================================
  Files           69       46      -23     
  Lines        10677     7152    -3525     
===========================================
- Hits          8655     5793    -2862     
+ Misses        2022     1359     -663     

see 27 files with indirect coverage changes

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[mitzimorris]

looks great!