Fix #1000 : support ItemSellerTradeParty in line level

library/src/main/java/org/mustangproject/Item.java

@@ -45,6 +45,7 @@ public class Item implements IZUGFeRDExportableItem {
 	protected String accountingReference;
 	protected String parentLineID = null;
 	protected String lineStatusReasonCode = null;
+ 	protected TradeParty lineSeller;
 	//protected HashMap<String, String> attributes = new HashMap<>();
 
 	/***
@@ -681,4 +682,19 @@ public class Item implements IZUGFeRDExportableItem {
 		this.lineStatusReasonCode = lineStatusReasonCode;
 		return this;
 	}
+
+	/***
+	 * For line seller 
+	 * @param seller The line seller
+	 * @return fluent setter
+	 */
+    public Item setLineSeller(TradeParty seller) {
+        this.lineSeller = seller;
+        return this;
+    }
+    @Override
+    public TradeParty getLineSeller() {
+        return this.lineSeller;
+    }
+
 }

library/src/main/java/org/mustangproject/ZUGFeRD/IZUGFeRDExportableItem.java

@@ -32,6 +32,7 @@ import java.util.List;
 
 import org.mustangproject.IncludedNote;
 import org.mustangproject.Item;
+import org.mustangproject.TradeParty;
 
 import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
 
@@ -237,4 +238,13 @@ public interface IZUGFeRDExportableItem extends IAbsoluteValueProvider{
 	}
 
     default LineCalculator getCalculation() {return new LineCalculator(this); };
+	/***
+	 * For line seller 
+	 * @return the seller
+	 */
+
+	default  TradeParty getLineSeller() { 
+		return null;
+	}
+
 }

library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java

@@ -525,8 +525,13 @@ public class ZUGFeRD2PullProvider implements IXMLProvider {
 					+ "</ram:ChargeAmount>" // currencyID=\"EUR\"
 					+ "<ram:BasisQuantity unitCode=\"" + XMLTools.encodeXML(currentItem.getProduct().getUnit())
 					+ "\">" + quantityFormat(currentItem.getBasisQuantity()) + "</ram:BasisQuantity>"
-					+ "</ram:NetPriceProductTradePrice>"
+					+ "</ram:NetPriceProductTradePrice>";
-					+ "</ram:SpecifiedLineTradeAgreement>"
+				  
+                if(currentItem.getLineSeller()!=null) {
+                    xml += "<ram:ItemSellerTradeParty>" + getTradePartyAsXML(currentItem.getLineSeller(), true, false) + "</ram:ItemSellerTradeParty>";
+    
+                }	            
+				xml += "</ram:SpecifiedLineTradeAgreement>"
 					+ "<ram:SpecifiedLineTradeDelivery>"
 					+ "<ram:BilledQuantity unitCode=\"" + XMLTools.encodeXML(currentItem.getProduct().getUnit()) + "\">"
 					+ quantityFormat(currentItem.getQuantity()) + "</ram:BilledQuantity>"