Sim Time initially negative due to last_dt being negative

[fred-huang122]

Describe the bug
When I print the sim time and last_dt in ssl_world.ccp: cout << "Sim Time: " << sim_time << "last dt: " << last_dt << "\n"; on line the sim time starts very negative and slowly becomes positive even if custom_DT is at 0.02. It seems that last_dt starts as -1 when the sim starts then adjusts to its intended value. I'm using (ubuntu 20.04). Thank you in advance :)