!!!API CHANGE!!! New Agent.act(...) method

Agent subclasses no longer return a vector from the act() method.
Agent's must now call Body subclass-specific methods to update
the agents desired velocities. biosim.app.tutorial.AvoidAnt
has a good example of this. Two drive interfaces are given in
biosim.core.body.actuators, and any new controller interfaces
will be located there in the future.

Future API changes: Any method that has the potential for being
called more than once per time-step during simulation (Agent.act(...),
sensors, actuators) will require a "current timestamp" as an
argument, so that multiple calls can be cached for composition
of behaviors. This change is planned in the next year or so
(2013-2014).