public void DemandsAdd(Demand demand)
 {
     if (demand.GetType() == typeof(ProductionOrderBom))
     {
         _productionOrderBoms.Add((ProductionOrderBom)demand);
     }
     else if (demand.GetType() == typeof(StockExchangeDemand))
     {
         _stockExchangeDemands.Add((StockExchangeDemand)demand);
     }
     else if (demand.GetType() == typeof(CustomerOrderPart))
     {
         _customerOrderParts.Add((CustomerOrderPart)demand);
     }
     else
     {
         throw new MrpRunException("This type is unknown.");
     }
 }