Note: The iterator system compiles and DeltaTransitionIterator
can be instanciated.

* include/vaucanson/automata/implementation/iterators/transition_iterator_base.hh,
* include/vaucanson/automata/implementation/iterators/transition_iterator_base.hxx:
Have weight() return a semiring_elt_value_t instead of semiring_elt_t.
Specialize iterator_traits_.
Add typename where missing.
* include/vaucanson/automata/implementation/iterators/delta_transition_iterator.hh,
* include/vaucanson/automata/implementation/iterators/delta_transition_iterator.hxx:
Fix iterator_traits_ to reflect changes in other files.
Add typename where missing.