Enhance SetBranchAddress so that the curent recommended code:

    chain->SetBranchAddres(branchname,&userdata); // once per chain
    ....
    branchptr = chain->GetBranch(branchname); // once per tree in the chain
can be replaced by:
    chain->SetBranchAddres(branchname,&userdata,&branchptr); // once per chain
As a result, branchptr will be either set to 0 when the branch does
not exist or the tree is not loaded yet and will be 'refreshed'
automatically everytime the chain loads a new tree.

MakeClass and MakeSelector have been updated to take advantage of
this new feature.  This removed most the code from the Notify
method.


git-svn-id: http://root.cern.ch/svn/root/trunk@15536 27541ba8-7e3a-0410-8455-c3a389f83636