add sample joss paper

[wang-boyu]

This is a first step for #90, using an example paper from JOSS: https://joss.readthedocs.io/en/latest/example_paper.html.

To build a pdf locally (assuming pandoc is installed with necessary dependencies):

cd paper/
pandoc --citeproc paper.md -o paper.pdf

JOSS uses a latex template but I couldn't get it working on my machine, so the resulting paper.pdf doesn't look the same as what JOSS produces. Nonetheless, this helps make sure that there isn't any compile error when generating the pdf.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@           Coverage Diff            @@
##             paper     #104   +/-   ##
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  Coverage         ?   91.19%           
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  Files            ?       14           
  Lines            ?     2193           
  Branches         ?        0           
========================================
  Hits             ?     2000           
  Misses           ?      193           
  Partials         ?        0           

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