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/expression/ExpressionProcessor.java
assert query != null;
		GroupSpec[] groups = query.getGrouping( );
		for ( int i = start; i < groups.length; i++ )
		{
			GroupSpec group = groups[i];
			if ( groupText.equals( group.getName( ) )
					|| groupText.equals( group.getKeyColumn( ) ) )
			{
				return i + 1; 
			}
		}
		return -1;
	}
	
	/**
	 * 
	 * @return
	 */
	private MultiPassExpressionCompiler getMultiPassCompilerHelper( )
	{
		if ( currentHelper == null )
		{
			currentHelper = new MultiPassExpressionCompiler( rsPopulator,
					baseQuery,
					dataset.getScriptScope( ),
					availableAggrList );
		}
		else
		{
			currentHelper.setCompilerStatus( availableAggrList );