Change cache behaviour
Change cache behaviour

Let V be a variable with extra paramaters,
that has been computed and stored in cache

If V is called another time without extra parameters,
 just return one of the values in cache

openfisca/openfisca-core,benjello/openfisca-core,openfisca/openfisca-core,benjello/openfisca-core,sgmap/openfisca-core