Add support for multiple periods in JSON of input variables and axes.
Add support for multiple periods in JSON of input variables and axes.

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