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
exprInfo = new ExpressionInfo( op1,
						exprType,
						currentGroupLevel,
						true );
				op1.setHandle( helper.compileExpression( op1, context ) );
			}
			if ( op2 != null )
			{
				exprInfo = new ExpressionInfo( op2,
						exprType,
						currentGroupLevel,
						true );
				op2.setHandle( helper.compileExpression( op2, context ) );
			}
		}
	}
	/**
	 * 
	 * @param baseExpression
	 * @throws DataException
	 */
	private void compileBaseExpression( IBaseExpression baseExpression,
			MultiPassExpressionCompiler helper ) throws DataException
	{
		Context context = Context.enter( );
		try
		{
			if ( baseExpression instanceof IConditionalExpression )
			{
				IConditionalExpression condition = (IConditionalExpression) baseExpression;