function _fnNodeToDataIndex( oSettings, n )
	{
		return (n._DT_RowIndex!==undefined) ? n._DT_RowIndex : null;
	}
Take a TR element and convert it to an index in aoData
 @param {object} oSettings dataTables settings object
 @param {node} n the TR element to find
 @returns {int} index if the node is found, null if not
 @memberof DataTable#oApi