added method to context/timer that allows agents to schedule early simulation termination for end of current timestep
adds prototype and implementation querying by name
fixed agent registration bug
added capability to have agents die and still 'produce' agents in the same timestep
only adds arc if preference is non-negative
errant semicolon
removed empty file
separated arc addition to its own function
added test
Moved RegisterAgent into DoRegistration method. made RegAgent methods private.

* Rather than having to remember to register agents everywhere we create
  instances, the DoRegistration method is for things that need to happen every
  time an agent instance comes to life - regardless if that was by db init or
  from new creation.  Moved RegisterAgent code out of all places and into
  Agent::DoRegistration.

* Fixed bug where Facility class didn't call Agent's DoRegistration method.

* Made [un]registerAgent methods in context private because the Agent class is
  already friends with the context and these methods should only ever be
  called by the kernel.
Added KillSim method and default solver to context (and some tests).

* If KillSim method on context is called one or more times, terminates
  simulation at the end of the current timestep.

* Added test that a bare simulation (no agents) - just an initialized timer
  and blank context run.

* If context's solver is null, it sets the solver to a default GreedySolver.

* Added test to check early simulation termination works.
Merge branch 'killsim' of https://github.com/rwcarlsen/cyclus into rwcarlsen-killsim
Merge branch 'rwcarlsen-killsim' into develop
Merge branch 'noarc' of https://github.com/gidden/cyclus into gidden-noarc
DoRegistration->EnterNotify
Merge pull request #838 from rwcarlsen/tweak

follow up to #836