Importer reads BillingSpecifiedPeriod of ApplicableHeaderTradeSettlement and SpecifiedLineTradeSettlement. Add ZugferdImporter#getDetailedDeliveryPeriodFrom and ZugferdImporter#getDetailedDeliveryPeriodTo. Fix extractIssuerAssignedID.
Importer reads BillingSpecifiedPeriod of ApplicableHeaderTradeSettlement and SpecifiedLineTradeSettlement. Add ZugferdImporter#getDetailedDeliveryPeriodFrom and ZugferdImporter#getDetailedDeliveryPeriodTo. Fix extractIssuerAssignedID.