Relations between users, groups and datasets

Aim of this user story is to define the relations between users, groups and model runs. Within this user story, we will develop a collaborative document that the scientists will comment on. 

The central questions are:

1. what relations are necessary to describe the relations between users, groups and model runs
2. what should they imply?

A first draft is written by @philipp.sommer, presented and discussed during one meeting and then sent out to the team for review.

related User Story (Hereon internal access only): #131