common: SIM_STATE specify attitude units as rad

[Maarrk]

The fields roll, pitch and yaw of SIM_STATE message do not specify their unit. Even if the recommended use is only for human-readable outputs, I believe it would be beneficial to have the unit specified.

Reasons for choosing radians:

• angular speeds of the same message are in rad/s
• consistency with ATTITUDE, MANUAL_SETPOINT, LOCAL_POSITION_NED_SYSTEM_GLOBAL_OFFSET, HIL_STATE, [GLOBAL_|VICON_]VISION_POSITION_ESTIMATE
• ArduPilot already sends them as rad (I don't have a PX4 setup ready to confirm)

Other units used for roll and pitch angles in common dialect:

• MAV_CMD_DO_SET_HOME - doesn't refer to attitude, but terrain
• MAV_CMD_DO_MOUNT_CONTROL, MOUNT_ORIENTATION - refers to gimbal and is deprecated, new messages use rad
• HIGH_LATENCY - deprecated
• MAV_CMD_NAV_TAKEOFF - degrees, but MAV_CMD_NAV_TAKEOFF_LOCAL uses rad
• Specifically yaw is often set in degrees in navigation context

[hamishwillee]

PX4 does not appear to implement this. If ArduPilot already sets this as radians then that is what we should do.

@julianoes @auturgy Could you please confirm this makes sense to you?

[julianoes]

Never seen this message, fine with me.

[hamishwillee]

OK, confirmed ArduPilot uses radians for these https://github.com/ArduPilot/ardupilot/blob/master/libraries/SITL/SITL.cpp#L1215 and PX4 does not care. So merging.