 - change default behavior of `seize` and `release` (release 4.0.0?, see issues #25, #27)
 - split resource into queue and server (release 4.0.0?)
 - split the package into core and graphics (release 4.0.0?)
 - start/stop a generator from inside a trajectory
 - implement a parallel branch activity
 - del_generator, del_resource?
 - add[del]_container (see simpy.resources.container)
 - add[del]_store (see simpy.resources.store)
 