[CSVDataset] Expose option to group the probes in SampleSets by reference id
This is necessary for the protocol "2.0.2" of bob.bio.face.database.frgc, in which probe reference templates are formed from a set of 4 probe images, which should be regrouped in a single SampleSet.
@tiago.pereira could you please have a look at it as we discussed it together yesterday ?