BigDecimal specific refactoring

Use BigDecimal.stripTrailingZeros() as key for getVATPercentMap and in consequence use compareTo... == 0 instead of equals
Use BigDecimal.ZERO instead of new BigDecimal(0)
These changes result in improvements in enumeration of the <ram:SpecifiedTradeAllowanceCharge> tags and the calculation of tag <ram:SpecifiedTradeSettlementHeaderMonetarySummation>.

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

@@ -16,7 +16,7 @@ public class Charge implements IZUGFeRDAllowanceCharge {
 	protected String categoryCode;
 
 	public Charge() {
-		taxPercent=new BigDecimal(0);
+		taxPercent=BigDecimal.ZERO;
 	}
 
 	/***