New Feature: Reset the Simulator

[Violin9906]

A new feature for resetting the simulator is introduced. You can reset the simulation without restarting it by simply publishing a bool message with true on topic /reset.
The behavior of some components are:

• timed: it will restart from time 0 and phase 0
• object manager: it will clear all stored objects
• scheduler: it will wait for all registered nodes to ready again, then send a message on /reset_time (which is required by TF2), then send the /start message
• mapd: it will publish the map again without reloading it
• traffic manager: it will respawn all traffic. Spawn points are regenerated
  The behavior of a car_agent can be customized by defining a __reset__ function to it.