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
if ( i < 0 )
			return null;
		if ( i == 0 )
			return currentRow;
		if ( i > 0 )
		{
			orderedArray.next( );
			return getNextMatchRow( orderedArray, values, dirs, sk );
		}
		return null;
	}
	/**
	 * Provide new sort keys for base IAggregationResultSet.
	 * @param base
	 * @param sortKeys
	 * @return
	 */
	private static List populateNewSortKeys( IAggregationResultSet base,
			SortKey[] sortKeys )
	{
		SortKey[] sks = new SortKey[base.getLevelCount( )];
		for ( int i = 0; i < sks.length; i++ )
		{
			for ( int j = 0; j < sortKeys.length; j++ )
			{
				int levelIndexInBase = base.getLevelIndex( sortKeys[j].getLevel( ) );
				if ( i == levelIndexInBase )
					sks[i] = sortKeys[j];
				if ( i < levelIndexInBase )
					break;