Optimize gear shifting for sucker chassis

[jwalthour]

Requested by @jch13

Desired shifting behavior:

• Defaults to high gear
• When robot runs up against significant resistance, shift down and activate suction

Detecting resistance could be done via current sensing, or possibly some other form of sensor.

[jwalthour]

#11 is a prerequisite for this issue

[jwalthour]

Sucker chassis outlawed

[lindsayvallen]

Since the season has gone in the crapper, we're looking at adding the sucker chassis despite the official ruling. However, we should still finish issue #11 first.