Update motor portion of loop to include start and end points.

[lwillmeth]

All good things must come to an end. Build in start and end points for the motor loop.

• [x] Do not turn on motors until Allison's apogee detection returns True.
• [x] Shut off the motors eventually. (Time delay? Parachute detection? Other?)

[lwillmeth]

The new state machine code in Avionics/Payloads/ (commit 4cc2ea) addresses this.