cubicweb-brainomics #3383893 Composite relations [validation pending]

We should add composite relations (and associated tests) for all measures/assessment/entities related to a subject.

prioritynormal
typebug
done in0.7.0
load1.000
load left0.000
closed by#aa98f6e98be8 [schema] Make relations composite where needed (closes #3383893).
patch[schema] Make relations composite where needed (closes #3383893). [applied]