cubicweb-compound #6304702 move CompositeGraph out of entities.py [done]

When there, it prevents its import in non-automatically-reloaded modules (e.g. schema.py, __init__.py, etc)

prioritynormal
typebug
done in<not specified>
load0.100
load left0.000
closed by#465699811eec Move CompositeGraph definition in __init__.py
patchMove CompositeGraph definition in __init__.py [applied]