GCS_MAVLink: add second mag to PREFLIGHT_SET_SENSOR_OFFSETS

Updates the comments for parameter 1 only, this does not affect the
message format.
GCS_MAVLink: generate after adding second mag to PREFLIGHT_SET_SENSOR_OFFSETS
GCS_MAVLink: add deprecated note to SET_MAG_OFFSETS
GCS_MAVLink: generate after adding deprecated note to SET_MAG_OFFSETS
Compass: save_offsets accepts compass instance

Only saves offsets for compasses that have initialised correctly
Compass: set_offsets modified to also save offsets
Compass: add learn_offsets_enabled accessor method
Compass: example sketch update for set_and_save_offsets
Copter: only save compass offsets when learning is on
Copter: PREFLIGHT_SET_SENSOR_OFFSETS used to set compass offsets

remove deprecated ardupilot specific SET_MAG_OFFSETS
Plane: rename compass set_and_save_offsets
Plane: PREFLIGHT_SET_SENSOR_OFFSETS used to set compass offsets

replaces ardupilot specific SET_MAG_OFFSETS message
Rover: rename compass set_and_save_offsets
Rover: PREFLIGHT_SET_SENSOR_OFFSETS used to set compass offsets

replaces ardupilot specific message SET_MAG_OFFSETS