Differentiate frames of reference

[linguini1]

The packet encoding should differentiate between frames of reference for measurements.

For example, linear acceleration from the IMU will be relative to the rocket's heading, whilst GPS acceleration measurements are relative to the earth.

Same for height, barometer altitude is in reference to the start position whilst GPS is in reference to sea level (although this is more easily corrected by taking a GPS measurement at ground level).

[linguini1]

Closed in #19