Add transmission support

[matthew-reynolds]

📚 Meta Issue

Support transmissions. This is most critical for mechanisms such as the drive train, where teams often have a 6 wheel skid-steer drive powered by 2 or 4 motors. For example, many teams would require a 2in-3out transmission (ideally with configurable reduction, allowing for multi-speed gearboxes) for their drive bases.

Tracking:

• Integrate transmissions into FRCRobotHW Use transmissions in RobotHW #2
• Finalize and document expected transmission syntax in YAML and URDF Specify transmission-related syntax in URDF and YAML #3
• Create custom X in, Y out transmissions Create X-input, Y-output transmission #4
• Support 2-speed transmissions Create 2-speed gearboxes #5

In order to maintain feature parity with simulation, this must also be implemented in Gazebo.