Four body example

[KaiHabermann]

Closes #37

Branch with notebooks, including:

• 4 body example showing non-trivial phi, theta and psi angles for the final state rotation
• 5 body example sampling phase space and showing histograms of the angles

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.63%. Comparing base (5f80f34) to head (300df29).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #41 +/- ## ======================================= Coverage 87.63% 87.63% ======================================= Files 6 6 Lines 631 631 Branches 107 107 ======================================= Hits 553 553 Misses 41 41 Partials 37 37 ```

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[mmikhasenko]

Nice! How long does it take to run? Would the notebooks be useful as a part of documentation.

If the running time is short, I'd be nice to run them in CI and reference HTML in the GitHub pages for the ## Examples section.

Meanwhile, I suggest removing cell output from this PR.

[mmikhasenko]

An example of extended conf.py that renders notebooks (feel like should be a separate PR since requires more work)

[KaiHabermann]

The notebooks take about a minute with the 1000x1000 grid. Going 500x500 should reduce this to 15 seconds, which may be fine. We could set the trigger for the render on manual or on release.