[py] Added zprobes and treferences to database

@tiago.pereira Could you confirm that I got it right:

zprobes:

• Returns a list of SampleSet objects (one for each subject)
• No references attribute (unlike probes).

treferences:

• Returns one list of SampleSet objects (one for each subject)
• Each SampleSet object has a cohort attribute that can be of any type, e.g. "M" for male (This is the part I'm not sure about... should I replace it by an int: e.g. male -> 0, female -> 1?)