function _fnGetUniqueThs ( oSettings, nHeader, aLayout )
	{
		var aReturn = [];
		if ( !aLayout )
		{
			aLayout = oSettings.aoHeader;
			if ( nHeader )
			{
				aLayout = [];
				_fnDetectHeader( aLayout, nHeader );
			}
		}
	
		for ( var i=0, iLen=aLayout.length ; i<iLen ; i++ )
		{
			for ( var j=0, jLen=aLayout[i].length ; j<jLen ; j++ )
			{
				if ( aLayout[i][j].unique &&
					 (!aReturn[j] || !oSettings.bSortCellsTop) )
				{
					aReturn[j] = aLayout[i][j].cell;
				}
			}
		}
	
		return aReturn;
	}
Get an array of unique th elements, one for each column
 @param {object} oSettings dataTables settings object
 @param {node} nHeader automatically detect the layout from this node - optional
 @param {array} aLayout thead/tfoot layout from _fnDetectHeader - optional
 @returns array {node} aReturn list of unique th's
 @memberof DataTable#oApi