Change FrontBack ultrasonic pair to be Front

[Seairth]

This might require extracting a common interface and creating a separate class for the FrontOnly variant. We will want to use this:

• When in teleop mode to tell use how far we are from objects.
• Part of an extended autonomy routine (see separate issue)

(note: we will want to update the dashboard to provide red/magenta indications of whether we are proper distance from Portcullis, Cheval, and Boulder. If possible, provide a relative distance required to be in position, such as "-4.5" to mean move back 4 1/2 inches to be the right distance away.)