Update TBranchElement::Unroll to no longer split a base class

that can not be split (i.e. respect the information returned
by TStreamerElement::CannotSplit (and thus TClass::CanSplit).

Add the ability to explicitly forbid (or allow) the splitting of a class
(TClass::SetSplit) so that user can inforce the use of a
custom streamer in all possible split cases.

This disabling is currently _not_ done automatically for
backward compatibility reasons and because
  a) Without TClass::SetCanSplit there was no way to 
     force the splitting (short of setting the split level lower)
  b) Some classes still requires a custom streamer solely to
     read older data files (for example for file written before
     the advent of StreamerInfo) and are such not necessary to 
     be used when writting (and schema evolution rules can not
     yet be used in this case).


git-svn-id: http://root.cern.ch/svn/root/trunk@39694 27541ba8-7e3a-0410-8455-c3a389f83636