Bug 182895 Support sort on Level attribute.
Currently in OLAP engine we only support sort on level key value. We should go ahead to support sort on level attribute value.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/olap/data/impl/aggregation/sort/AggregationSortHelper.java
/*
	 * (non-Javadoc)
	 * @see org.eclipse.birt.data.engine.olap.data.util.IDiskArray#add(java.lang.Object)
	 */
	public boolean add( Object o ) throws IOException
	{
		assert o instanceof CombinedAggrResultRow;
		CombinedAggrResultRow obj = (CombinedAggrResultRow) o;
		AggregationResultRow baseRow = new AggregationResultRow( );
		baseRow.setLevelMembers( obj.getLevelMembers( ) );
		this.base.add( baseRow );
		AggregationResultRow keyRow = new AggregationResultRow( );
		keyRow.setAggregationValues( obj.getAggregationValues( ) );
		this.aggrValues.add( keyRow );
		return true;
	}
	/*
	 * (non-Javadoc)
	 * @see org.eclipse.birt.data.engine.olap.data.util.IDiskArray#clear()
	 */
	public void clear( ) throws IOException
	{
		this.base.clear( );
		this.aggrValues.clear( );
	}
	/*
	 * (non-Javadoc)
	 * @see org.eclipse.birt.data.engine.olap.data.util.IDiskArray#close()
	 */
	public void close( ) throws IOException