Create a Tutorial on the Grand-Reaction Method

[pm-blanco]

I think it would be a good idea to have a separate tutorial on the grand-reaction method, since it involves a couple of subtle points that one needs to be aware of when using the method. One could for example also explain the two different implementations (original and Luijten) that will be provided.

Original issue by @davidbbeyer migrated from our original remote repository on Gitlab, see here for further discusion on the topic here