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
/**
		 * Get the next accumulator. If there is no next accumulator, populate one.
		 * @return
		 */
		Accumulator next( )
		{
			if( this.accumulator!= null )
				return this.accumulator;
			cursor++;
			if( cachedAcc.size() > cursor )
			{
				return (Accumulator)cachedAcc.get(cursor);
			}else
			{
				cachedAcc.add( aggregation.newAccumulator() );
				return (Accumulator)cachedAcc.get(cursor);
			}
		}
		
		/**
		 * Reset the cursor to unstart state ( = -1)
		 *
		 */
		void restart( )
		{
			this.cursor = -1;
		}
	}
}