Create X-input, Y-output transmission

[matthew-reynolds]

🚀 Feature Request

See #1. ros_control currently only provides very basic transmissions. We require a generic custom transmission that allows for a variable number of inputs and outputs to be linked. A typical use case would be drive trains; A 4-CIM 6-wheel skid steer would require a 2-in 3-out transmission for each side of the drive base.

Typically, all inputs will be equal, and all outputs will be equal, with a specific reduction between them. (Think WCP gearboxes). We may want to support having independant gear reduction for each input and output, though this is not a priority and would likely be more involved.