Motor output from MBED

[oswinso]

Description

We currently have no idea what values our MBED is sending to the motors, which is kind of scary, and hard to debug controls problems.

This PR does the following:

• Creates a igvc_msgs::motor_control message type for modelling the MBED output to the motors (0 - 255)
• Updates the igvc.pb.c and igvc.pb.h to include the new left_output and right_output fields
• Updated motor_controller to extract the motor outputs and publish them.

Fixes #403.

Testing steps (If relevant)

Test on Jessii

1. Launch motor_controller on Jessii and a joystick node, with wheels decoupled
2. rostopic echo /motor_output
3. Move the joystick around

Expectation: It should values corresponding to what you are joysticking

Self Checklist

• [x] I have formatted my code using make format
• [ ] I have tested that the new behaviour works