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 aggregate value
	 * @param aggrIndex
	 * @return
	 * @throws DataException
	 */
	public Object getAggrValue( String name ) throws DataException
	{
		IAggrInfo aggrInfo = this.manager.getAggrDefn( name );
				
		if ( this.populator.getCache( ).getCount( ) == 0 )
		{
			if ( aggrInfo.getAggregation( ).getName( ).equalsIgnoreCase( IBuildInAggregation.TOTAL_COUNT_FUNC)
				 ||	aggrInfo.getAggregation( ).getName( ).equalsIgnoreCase( IBuildInAggregation.TOTAL_COUNTDISTINCT_FUNC))
				
				return new Integer( 0 );
			else
				return null;
		}
		try
		{
			int groupIndex;
			if ( aggrInfo.getAggregation( ).getType( ) == IAggregation.SUMMARY_AGGR )
			{
				
				if ( aggrInfo.getGroupLevel( ) == 0 )
					groupIndex = 0;
				else
					groupIndex = this.getCurrentGroupIndex( aggrInfo.getGroupLevel( ));
			}
			else