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
*/
		AccumulatorManager( IAggregation aggregation )
		{
			this.aggregation = aggregation;
			this.cursor = -1;
			
			int passNum = 0;
			if( aggregation instanceof Aggregation )
				passNum = ((Aggregation)aggregation).getNumberOfPasses( );
			if( passNum < 2 )
				this.accumulator = aggregation.newAccumulator();
			else
				this.cachedAcc = new ArrayList();	
		}
		
		/**
		 * Get the current accumulator. 
		 * @return
		 */
		Accumulator getCurrentAccumulator( )
		{
			if( this.accumulator!= null )
				return this.accumulator;
			if( cachedAcc.size() == 0 )
			{
				cachedAcc.add( aggregation.newAccumulator() );
			}
			return (Accumulator)cachedAcc.get(cursor);
		}