Add test for threads parameter

[odow]

x-ref #186

Closes #112

[codecov]

Codecov Report

Merging #190 (d8c508f) into master (b2cd21f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #190   +/-   ##
=======================================
  Coverage   61.61%   61.61%           
=======================================
  Files           3        3           
  Lines         581      581           
=======================================
  Hits          358      358           
  Misses        223      223           

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