Updated param and imagen submodule references
Simulation now uses the param.Dynamic.time_fn for time

Simulation and all other objects with time functions now use
param.Dynamic.time_fn. By default this is a Time object, which offers
functionality which may be accessed via topo.sim.time, such as the
ability to set a soft run limit with topo.sim.time.until. The bug
where topo.sim.time can be made to jump backwards has also been fixed.