Overload the `get` function of `Requests` so that, in addition to the
usual `Mapping` behavior, will return an empty `ObjectSet` if no default
is specified.
Improve the `schedule` in the `Pipeline` class by introducing two
optimizations:
* Prune all the `ContainerDeclarations` that are not actually used.
* Skip all tasks that have no outgoing elements that would have been
written by them.
Giacomo Vercesi