Code cleanup.
Rework Scenario JSON.

Now the JSON of an API request looks like:
dict(
    scenarios = [
        dict(
            familles = [
                dict(
                    parents = ['ind0', 'ind1'],
                    enfants = ['ind2'],
                    ),
                ],
            foyers_fiscaux = [
                dict(
                    declarants = ['ind0', 'ind1'],
                    personnes_a_charge = ['ind2'],
                    ),
                ],
            individus = [
                dict(
                    birth = '1965-12-27',
                    id = 'ind0',
                    sali = 12000,
                    ),
                dict(
                    birth = '1965-12-28',
                    id = 'ind1',
                    ),
                dict(
                    birth = '2005-12-29',
                    id = 'ind2',
                    ),
                ],
            menages = [
                dict(
                    personne_de_reference = 'ind0',
                    conjoint = 'ind1',
                    enfants = ['ind2'],
                    ),
                ],
            year = 2006,
            ),
        ],
    )
Signal an error when an individual doesn't belong to a famille, a foyer_fiscal or a menage.
Fix conflict
Fix conflict II
Fix conflict III
Update OF IPP parameters