Importations xscen about xhydro

[Zeitsperre]

One thing worth noting (I'm going to sound like a broken record to @RondeauG) is that if we make xscen an installation dependency, this will pull in all the dependencies that xscen depends upon. From the scope of xhydro, it's very possible that there's going to be a fair amount of overlap, but it's worth considering as well.

For creating the bridge between xhydro and xscen, the documentation for the intersphinx plugin can be found here: https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html