Convert docstring plot directives to doctests for strat.py

[mcflugen]

This pull request converts the plot directives in the strat.py module to doctests.

ref: #170

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.27%. Comparing base (b6289e2) to head (ad2f47e).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #177      +/-   ##
===========================================
+ Coverage    89.16%   89.27%   +0.11%     
===========================================
  Files            9        9              
  Lines         3617     3617              
===========================================
+ Hits          3225     3229       +4     
+ Misses         392      388       -4     

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