Add tutorial for the new functionality of dataset automatic creation from files
   created on the workers, using TProofOutputFile. 
   The tutorial is referred to as "dataset" in runProof.C and uses the selector
   ProofNtuple as the "ntuple" tutorial, with a slightly different configuration.
   The created dataset is called 'testNtuple' and it is automatically registered
   and verified. The dataset is then used to produce the same plot as in "ntuple"
   but using the DrawSelect methods of PROOF, showing how to set style, color and
   other drawing attributes in PROOF.

   To try it out, do

          root [0] .L tutorials/proof/runProof.C+
          root [1] runProof("dataset")


git-svn-id: http://root.cern.ch/svn/root/trunk@30183 27541ba8-7e3a-0410-8455-c3a389f83636