From Gerri:

 Attached is the patch to  improve the default speed of TChain::SetProof.

I have added two flags:
  - to control whether the tree header has to be retrieved or not
   (default kFALSE).
  - to force refreshing of the associated fChainProof; default is kFALSE;
    this is used internally by the "Add..." methods if fChainProof is defined,
    so that the new additions to the chain are automatically taken into account
    in the associated data set (this was one of the problems Jan-Fiete had).

TChainProof::StartViewer now checks if the tree header is available and,
if not, tries to get it from PROOF; so the behaviour is always the same,
except that now one waits only when the viewer is started, and not for
normal processing.


git-svn-id: http://root.cern.ch/svn/root/trunk@15680 27541ba8-7e3a-0410-8455-c3a389f83636