fix jerk limiter in diff_drive_controller

[tpoignonec]

There seem to be an error in the jerk limiter (speed_limiter.cpp#L117-L134) caused by the (as far as I can tell) incorrect factor 2 in the computation of the velocity limit resulting from jerk limits.

The consequence is an overshoot of the configured jerk limit.
To illustrate the effect of this factor, let us set the linear jerk limit as $J_{max} = 20 \text{ m.s}^{-3}$ such that

diffbot_base_controller:
  ros__parameters:
    type: diff_drive_controller/DiffDriveController
    ...

    # Velocity and acceleration limits
    # Whenever a min_* is unspecified, default to -max_*
    linear.x.has_velocity_limits: true
    linear.x.has_acceleration_limits: true
    linear.x.has_jerk_limits: true
    linear.x.max_velocity: 1.0
    linear.x.min_velocity: -1.0
    linear.x.max_acceleration: 2.0
    linear.x.min_acceleration: -5.0
    linear.x.max_jerk: 20.0
    linear.x.min_jerk: -20.0

    ...

Before this PR, the jerk reaches $40 \text{ m.s}^{-3}$:

After this PR:

[christophfroehlich]

Thanks for reporting.

There is an ongoing process in moving this to control_toolbox
#1315
Could you please adding your PR there?

[tpoignonec]

Was moved to control_toolbox (#240).