Respect speed limit in csc file when headless

[pjonsson]

Switch the default behavior of Cooja to respect the specified speed limit in headless mode. The previous behavior is still available by passing the ignore-speed-limit key-value pair to -nogui:

-nogui=file.csc,ignore-speed-limit=true

[nfi]

Instead of changing the default behavior for Cooja when running without GUI - what do you think about using the argument speed-limit to tell Cooja to respect the speed limit configured in the simulation file?

I think most would like Cooja to run with max speed when running headless unless there are specific reasons not to, for example, if the simulation communicates with external actors.