Time series in simulation should reference simulation time instead of wall clock

[Flova]

This had been implemented for ROS1 as discussed here: PlotJuggler/PlotJuggler#273 But it somehow got lost/not implemented for ROS2. This leads to awkward jumps in the plot if you stop the sim or skewed slopes if the sim runs faster/slower.

[facontidavide]

I will check this out...