add small templated TObjNum class. Version for all basic types are available

as TObjNumC, TObjNumS, TObjNumI, etc. Altough not very memory efficient
(since containing a TObject in addition to the basic type) it is useful in
some circumstances, like TObjString.


git-svn-id: http://root.cern.ch/svn/root/trunk@5737 27541ba8-7e3a-0410-8455-c3a389f83636