Provide the option for smooth stopping for arm motions (e.g., trapezoidal velocity profile)

[hello-amal]

As of #144 , the default stopping behavior is immediate stopping. This is good from a safety perspective, but the sudden deceleration can be jarring for users (e.g., due to the sound, as well as the resulting jerkiness it might cause for any object the robot is holding).

This issue focuses on adding smooth stopping behavior (e.g., with a trapezoidal velocity profile), where the steepness of the trapezoid is controlled by a parameter to stretch_driver. By setting that parameter to inf (which can perhaps be the default) we can get the current immediate stopping behavior, but setting it to lower values can lead to smoother declaration. That way, users of stretch_driver can decide their desired stopping declaration depending on their application.