Gripper: Quadropus

Now it has a upgraded version, see [Quadropus v2](https://www.thingiverse.com/thing:6782188)
A gripper designed for [UMIRobot](https://www.thingiverse.com/thing:4797804).
Also see [Master: Box](https://www.thingiverse.com/thing:6107851).
This work has been published to [IROS 2023](https://ieee-iros.org).
Refer our paper: [UMIRobot: An Open-{Software, Hardware} Low-Cost Robotic Manipulator for Education](https://doi.org/10.48550/arXiv.2301.06668).