always returns an already constructed object.   If the slot is being used for the
first time, it calls the default constructor otherwise it returns the object as 
is (unless a string is passed as the 2nd argument to the function in which case,
it also calls Clear(second_argument) on the object).
This allows replace code like:
   for (int i = 0; i < ev->Ntracks; i++) {
       new(a[i]) TTrack(x,y,z,...);
       ...
       ...
   }
   ...
   a.Delete(); // or a.Clear("C")
with the simpler and more efficient:
   for (int i = 0; i < ev->Ntracks; i++) {   
       TTrack *track = (TTrack*)a.ConstructedAt(i);
       track->Set(x,y,z,....);
       ...                                                          
       ...                                                          
   }                                                               
   ...                                                             
   a.Clear();
even in case where the TTrack class allocates memory.


git-svn-id: http://root.cern.ch/svn/root/trunk@40562 27541ba8-7e3a-0410-8455-c3a389f83636