Contains all the router classes who decide
how to handle the messages. All router classes must be in this package
and must extend the {@link routing.MessageRouter} (when creating new
routers, extending {@link routing.ActiveRouter} might make sense) class
so they can be dynamically loaded to the simulator. The classes to load
can be specified through {@link core.Settings} class' settings source.
See MessageRouter class and classes extending it for details about the
settings.