Don't fail when creating a FormulaColumn with a reference that is a column without formula.
Don't fail when creating a FormulaColumn with a reference that is a column without formula.

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