Updates the PO cost quantity and linecost based on the percentage entered. 
 If the PO line quantity is null, then the percentage applies to the linecost,
 otherwise it applies to the quantity. While calculating these values, PO cost 
 lines totals are kept equal to the PO line values.
 Conditions :
 
      - If the sum of the percentages is 100, 
                        set the quantity on the PO cost line  as the difference between the 
                        quantity on the PO line and the sum of quantities on the other PO cost lines;
                set the linecost as the difference between the linecost on the PO line and 
                        the sum of the PO cost linecosts of  the other POcost lines
                set the loadedcost as the difference between the loadedcost on the PO line and 
                        the sum of the PO cost loadedcost of  the other POcost lines
     
-        If the sum of the percentages is not 100,
                        set the PO cost quantity = quantity * percentage/100;
                set the POcost linecost=linecost * percentage/100