Provide TIter &TIter::operator=(TIterator *it) signature

[linev]

Copy post from Mattermost:

Normally one writes:

TList *lst = new TList(); 
TIter iter(lst);

This works fine.

In many places of RooFit (and also in some other classes) one can see following syntax:

TIter iter = lst->MakeIterator();

It is also fine while where is constructor signature TIter(TIterator *it).
But if one does again:

 iter = lst->MakeIterator();

One do not get that one expects. While C++ does:

1. creates temporary TIter instance,
2. Calls TIter &operator=(const TIter &rhs)
3. deletes temporary TIter instance with original TIterator object which was created by lst->MakeIterator().

Means we have unnecessary duplication of TIterator in between. Moreover, following code MAY produce wrong results:

  iter = lst->MakeIterator(kIterBackward);

It depends if assign operator implemented properly for derived TIterator classes.
Probably, we should define assign operator abstract:

virtual TIterator &operator=(const TIterator &) = 0;

To ensure that all derived classes implement it