Here is an example: "4,3,0,1:6,5,1-3,7-". It specifies to use fields 4,3,0 and 1 for keys,
 * and use fields 6,5,1,2,3,7 and above for values.
Here is an example: "4,3,0,1:6,5,1-3,7-". It specifies to use fields 4,3,0 and 1 for keys,
 * and use fields 6,5,1,2,3,7 and above for values. | FieldSelectionMapReduce.java