Specify transmission-related syntax in URDF and YAML

[matthew-reynolds]

🚀 Feature Request

See #1. While the syntax for transmissions themselves is well defined in URDF, we need to establish the appropriate syntax for sensors linked to joints vs actuators. We also need to handle multi-speed transmissions, and therefore the associated actuator that shifts gears (Typically a solenoid for 2 speed gearboxes. What about more than 2 speed?)