Avionics - Add methods to calibrate sensors

[lwillmeth]

We need to add calibration methods for the following sensors:

PCF (real time clock):

• [ ] Get time from internet
• [ ] Set linux clock

MPU9250, can store offsets in sensor registers:

• [ ] Zero out accelerometers
• [ ] Zero out gyros

MPL Altitude sensor, may need to take ground pressure and altitude as inputs:

• [ ] Convert pressure to altitude (in feet)

[lwillmeth]

Addressed by e89938a8a233848fbc50bd59b8d5ba6252aa2ef7