Move TaxBenefitSystem class in its own module.
Move TaxBenefitSystem class in its own module.

adrienpacifico/openfisca-france,adrienpacifico/openfisca-france,benjello/openfisca-france,benjello/openfisca-france,SophieIPP/openfisca-france,sgmap/openfisca-france,SophieIPP/openfisca-france,sgmap/openfisca-france,antoinearnoud/openfisca-france,antoinearnoud/openfisca-france