Not all commands always need dependencies (pri, depri, ls -x), but all
administration is still built up every time a TodoList is instantiated.
Now, as soon as something dependency related is requested from the
TodoList, the dependency graph and caches are initialized.

A decorator is used to mark methods in TodoList that require an
initialized graph.

This change impacts the calculation of the average importance (based on
the todo's ancestors). Importance doesn't have a notion of TodoList (and
I would like to keep it that way), so the calculation needs access to
the parents somehow. This was done with a parents attribute stored with
each Todo item, carefully kept up to date by the TodoList. This is
cumbersome, instead, now a Todo item gets a parents() method that
obtains the parents from the TodoList that keeps the todo item.