Revert "Allow SimpleFormula function to return a scalar number"
Revert "Allow SimpleFormula function to return a scalar number"

This reverts commit e4baeb0891c47ecc20d646a760b39dcc5a64fcfe.

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