Update dataset.py
Update dataset.py

OK, I think I could clean up the logic but then I am not sure it will be clear what I am attempting.  I leave for Ben to review.
T2DREAM/t2dream-portal,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,T2DREAM/t2dream-portal,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,ENCODE-DCC/encoded,ENCODE-DCC/encoded