Provide monitor command to abort current step

[thomaspeugeot]

Situation:

• with a simulation with more than a million bodies, a step can take more than 5 minutes.
• when the simulation is running on a laptop, the laptop is frozen since all CPU are utilized

Problem:

• If one want to do something else, one have to wait for up to 5 minutes

Solution:

• have a command that abort the current step.
• distinguish simulation states between "RUNNING" and "RUNNING_RESQUETED" as well as "STOPPED" and "STOP_REQUESTED"