This patch implements a const Write and keeps the existing non-const
Write. If somebody derived from TObject and overload Write(), they
will be reminded to implement the const version with a warning
similar to:

include/TCollection.h(45): warning #654: overloaded virtual function "TObject::Write" is only partially overridden in class "TCollection"
 class TCollection : public TObject {

The implementation of the non-const Write is simple: call the const Write().

Exceptions are TFile and TDirectory which actually can only implement
non const Write (and hence const Write outputs an error message).


git-svn-id: http://root.cern.ch/svn/root/trunk@9619 27541ba8-7e3a-0410-8455-c3a389f83636