Functional and non-functional aspects
From the GUI, there should be a way to add relations between
requirements.
The “all functional aspect is related to a non-functional one” needs to
use this.
The constraint should not check that a requirement has two aspects, but
rather the model needs to be modified to keep track of the relations
(from redmine: issue id 3132, created on 2017-11-03, closed on 2018-03-05)