Consider renaming default branch from master to main

This is not something that is urgent at all but we probably want to look at it for v2.

I am putting it here to avoid comments from random strangers in our public repo since this is potentially a topic that attracts trolls …

This will probably break some things in FUN, but not that sure what needs to be updated, maybe only the logic that creates the user environment with all the notebooks and maybe the “reset to original” fonctionality?

Done!