Bug 189706 Enhance computed column aggregation function handling in UI.
Due to the issue discussed in 189705, current computed column aggregation builder in data set editor has bugs to extend: 1.When new a computed column, the aggregation argument field cannot be correctly set. 2.The argument field cannot be correct grayed out when user select a aggregation function without arguments 3.The available aggregation function list is incomprehensive 4.Some aggregation function cannot be set to model(say, weightedAve).
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/executor/aggregation/AggregationHelper.java
{
				groupIndex = this.getCurrentResultIndex( );
			}
			return this.currentRoundAggrValue[this.manager.getAggrDefnIndex( name )].get( groupIndex );
		}
		catch ( DataException e )
		{
			throw e;
		}
	}
	
	public boolean hasAggr( String name ) throws DataException
	{
		return this.manager.getAggrDefnIndex( name ) != -1;
	}
	
	/**
	 * A helper class that is used to manage the Accumulators of aggregations. 
	 *
	 */
	private class AccumulatorManager{
		
		private IAggregation aggregation;
		private int cursor;
		private List cachedAcc;
		private Accumulator accumulator;
		
		/**
		 * Constructor.
		 * @param aggregation