Documentation

[lukasmerten]

This PR includes mainly updates of CRPropa's documentation, e.g.:

• A completely new example for custom photon fields
• updates to most of in-code documentation that is used by doxygen, too
• clean-up of deprecated methods in example notebooks
• removing long deprecated features
• updating links to recent CRPropa publications
• etc.

While preparing this PR some small fixes were added to the code.

I will upload a recent version of the documentation into a cloud for checking purposes in the next days and link it here.

Likely some smaller changes will be made in the next days, therefore the PR stays in draft version for some time. However, reviewing can start now.

Edit: 230209: This should also close #411. 230210: You can take a look at the documentation and coverage report here 230412: This PR also removes tests on Ubuntu 18.04 as they seem to be finally not supported anymore. See e.g., here for a list of supported runners.

[lukasmerten]

@mohller and @rafaelab thanks for reviewing this PR.

Is there anything else you want to be addressed or something we missed to update? Otherwise please approve the PR so we can finally roll out the new documentation.

[rafaelab]

Is there anything else you want to be addressed or something we missed to update? Otherwise please approve the PR so we can finally roll out the new documentation.

I think we are good to go. There will certainly me more things, but that can be fixed later. Once again, many thanks to all of you who worked on the documentation.

[mohller]

Some minor comments and suggestions

Sorry for the late reply, I found no issues and had only small suggestions. Thanks for the great work!

[lukasmerten]

Thanks to everyone who was involved in this work on the documentation. I think we made quite a step forward to improve the usability of CRPropa.