Lifter arms commands

[wilstenholme]

Let's work on this after basic structure and pot values are good from merging #113.
We would like some commands to move the arms to some certain positions, using pot values.

[rtfoley]

We'll want to think about how fine of control do we need and what special situations do we have. For example, it might be good to have a quick-retract button so the arms can be brought it quickly when going under the low bar. We will have to talk with the driver about whether we can handle the defenses with digital control, or if we need analog.

[BenBernardCIS]

A couple of notes on this:

1. With the backdriveable motor, we definitely need more active control (otherwise the arms just drift back down to the floor)
2. It's still hard to time the lifting and traversing. We need the arms commands to be usable as units of a "driver assist" routine.

[wilstenholme]

From #113,

We need constants for potentiometer value at:

• Stowed position
• Straight up position
• Hall effect sensor (possibly a range)
• On the floor when robot is flat
• On the defense when robot is entering portcullis
• Holding the portcullis clear of the robot
• Just above the Cheval when approaching the Cheval
• Holding the cheval down