Planner steps are given in absolute time, not simulation step

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Planners are written as a series of planner implementations with each planner having a monotonic step, where a planner becomes active after the simulation step is greater than or equal to the listed step.
In the mainline Peng implementation, the control and simulation frequencies were fixed such that each tick was equal to 1ms.

For ease of planner development, we will want the active times for each step to be an absolute simulation time since simulation epoch. This saves us from having to back out the step number from the desired time and simulation frequency.