Wrong integration of velocity feedback in diff_drive_controller

[roncapat]

See #260 (comment)
Thanks to @wmmc88 for spotting this bug.

[ros-gy]

Would the fix for this issue to change file diff_drive_controller.cpp, line 220/221 (depending on master/galactic branch) to:

odometry_.updateFromVelocity(left_feedback_mean*update_dt.seconds(), right_feedback_mean*update_dt.seconds(), current_time);