Modify control law to accept Twist as input

[mbr4477]

Control law will ignore the angular component.

[mbr4477]

Some clarification:

Currently, control.py contains both the nav and the control law for moving in a desired heading. It outputs a custom motor message that is picked up by motor.py.

In the new architecture, control.py will be split up. The ideal nav portion (i.e., heading to destination calculation, will be in its own node. The desired heading to motor speeds will also be in its own node. In between these nodes will be one that does the obstacle avoidance (adjusting the Twist published by the ideal nav into a modified Twist). Finally, the control law (desired heading to motor speeds) will accept this modified Twist and convert it to a motor_speeds message for the motor.py node.

[ruffner]

@jashley2017 this looks similar to what you just implemented for the demo bots