Gripper: Implement proper handling of open-loop gripper control

[sbstorz]

Problem
The hobby RC servo used to actuate the gripper has no position feedback. The reported state feedback is simply the last command. This means that the gripper position immediatly returns successfull execution after a new set point has arrived. However the gripper has a finite speed and needs around 0.5 s to actuate and go to its new position. But if a trajectory directly continues after a gripper command, the gripper has no time to finish its movement. b6bc644 has implemented a hotfix to delay the reported success, however it really is just a rough workaround.

Solution
Either:

1. Implement a digital twin that internally estimates the motion of the gripper and returns estimated position feedback
2. Deploy a velocity limited controller as described in Gripper Trajectory #26, one can then possibly assume for infintly small changes that the the gripper acts immediatly.