Up depth of models
As LDFlex have some issues with performances, we could try to help it handling datas by raising depth of our models.
For testing, we have to use the currently-in-dev version of sib-core (branch bugfix/performances-ldflex, may be published soon)
- Should we set default depth of djangoldp higher than 0? That could mean that we don't have to change our models but only djangoldp to make it works.
- Decide which depth by model? More than 1 could give some redundancy:  Circle ->  Owner (User) ->  Circles (Circle)