JOSS review changes

[robashaw]

This addresses issues #34, #35, #36, and #37 created as part of the JOSS review process. It contains the JOSS paper, along with the following changes:

• code of conduct and architecture files
• documentation of auxiliary python scripts
• default build Release (with optimizations turned on)
• a standalone example, separate from the testing suite

The python bindings mentioned in issue #8 are being handled separately.

This branch will be merged at the end of the review process.

[codecov]

Codecov Report

Merging #38 (035a3de) into master (d53a409) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   89.71%   89.80%   +0.09%     
==========================================
  Files          19       19              
  Lines        1954     1952       -2     
==========================================
  Hits         1753     1753              
+ Misses        201      199       -2     

Impacted Files	Coverage Δ
include/libecpint/ecpint.hpp	100.00% <ø> (ø)
src/lib/api.cpp	81.27% <0.00%> (+0.68%)	⬆️

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[felipeZ]

Great job!