Modify how simulations end

[stvdwtt]

Currently the simulation runs until a designated end time. We discussed changing this so that the simulation end time is optionally dictated by the scan path. This requires extending the syntax of the scan path files to include something that denotes the end of the simulation. (This is needed for dynamic changes to the scan path in #223 to disambiguate between a simulation that is waiting for more scan path info and a simulation ready to be completed.)