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/impl/ComputedColumnHelper.java
* @see org.eclipse.birt.data.engine.odi.IResultObjectEvent#process(org.eclipse.birt.data.engine.odi.IResultObject,
	 *      int)
	 */
	public boolean process( IResultObject resultObject, int rowIndex )
			throws DataException
	{
		if ( this.getCurrentInstance( ) != null )
			return this.getCurrentInstance( ).process( resultObject, rowIndex );
		else
			return true;
	}
	/**
	 * Return whether the computed column set with given model exists
	 * 
	 * @param model
	 * @return
	 */
	public boolean isComputedColumnExist( int model )
	{
		if ( model == TransformationConstants.DATA_SET_MODEL )
			return this.dataSetInstance.getComputedColumnList( ).size( ) > 0;
		else if ( model == TransformationConstants.RESULT_SET_MODEL )
			return this.resultSetInstance.getComputedColumnList( ).size( ) > 0;
		else if ( model == TransformationConstants.ALL_MODEL )
			return this.allCC.size( ) > 0;
		return false;
	}
	/**
	 * Return a list of computed column of current instance.
	 *