katana: remove acceleration discontinuity warning

The trajectories generated by arm_navigation and MoveIt have
acceleration discontinuities, and that's fine, so this warning has to be
removed. As far as I can tell, the constraints are:

* positions have to be continuous and differentiable
* velocities have to be continuous
* accelerations don't have to be either
remove "Trajectory started to late" warning if start_time == 0

Moveit usually doesn't set the start time.