Removed old debugging/obsolete code from IMU node and primary node code.
Replaced some byte-unpacking code with the nice new functions from Packing.h
NMEA2000 lib: Added decoding of MAGNETIC_VARIATION message, PGN127258.
Primary node: altered code so that it references a generic IMU and not the REVO_GS. Also added proper detection for IMU based on received CAN messages.