Fix datatype of PaymentMeansCode in Rules

Fix datatype of PaymentMeansCode in Rules. E.g. BR-DE-19 for UBL is currently /ubl:Invoice/cac:PaymentMeans[cbc:PaymentMeansCode = (30,58)] | /cn:CreditNote/cac:PaymentMeans[cbc:PaymentMeansCode = (30,58)], but the numbers have to be strings.