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)

[sappelhoff]

Note that regarding the validation part of the example, we should also rely on @a-hurst's https://github.com/a-hurst/matprep_artifacts/tree/master

instead of the old saved data. As for some reason, things have become out of sync and it looks terrible now: https://pyprep.readthedocs.io/en/latest/auto_examples/run_full_prep.html#sphx-glr-auto-examples-run-full-prep-py