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
* compile conditional expression
	 * 
	 * @param baseExpression
	 * @param parser
	 * @throws DataException 
	 * @throws DataException
	 */
	private void compileConditionalExpression(
			IConditionalExpression baseExpression,
			MultiPassExpressionCompiler helper, ResultSetPopulator rsPopulator,
			int exprType, int currentGroupLevel, Context context ) throws DataException
	{
		ExpressionInfo exprInfo = null;
		if ( baseExpression instanceof IConditionalExpression )
		{
			IConditionalExpression condition = (IConditionalExpression) baseExpression;
			IScriptExpression op = condition.getExpression( );
			IScriptExpression op1 = condition.getOperand1( );
			IScriptExpression op2 = condition.getOperand2( );
			if ( op != null )
			{
				exprInfo = new ExpressionInfo( op,
						exprType,
						currentGroupLevel,
						true );
				op.setHandle( helper.compileExpression( exprInfo,
						context ) );
			}
			if ( op1 != null )
			{