Add example of eigenfunction of Koopman operator

[blegat]

Preview https://jump-dev.github.io/SumOfSquares.jl/previews/PR270

• The plot does not correspond to the one of the book and the error is large, need to figure out what's wrong

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.46%. Comparing base (9770cfb) to head (efa3fd0).
⚠️ Report is 87 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #270   +/-   ##
=======================================
  Coverage   78.46%   78.46%           
=======================================
  Files          36       36           
  Lines        1788     1788           
=======================================
  Hits         1403     1403           
  Misses        385      385           

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