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[bot]]

Codecov Report

Base: 78.46% // Head: 78.46% // No change to project coverage :thumbsup:

Coverage data is based on head (efa3fd0) compared to base (9770cfb). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #270 +/- ## ======================================= Coverage 78.46% 78.46% ======================================= Files 36 36 Lines 1788 1788 ======================================= Hits 1403 1403 Misses 385 385 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev)

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