Split "full PREP example" into two: 1) show how to use, 2) show validation against matlab

[sappelhoff]

Currently the full PREP example does both, showing how it can be used, and validating the results against the Matlab results.

https://pyprep.readthedocs.io/en/latest/auto_examples/index.html

I think we should rather have two examples:

• one example where PREP usage is shown more extensively and with more explanations than currently
• and another example with a focus on validation against the Matlab results (here, the plotting code should be improved, because currently it's very messy to read and probably lots of unneeded code duplication)