Enable user defined and JSBSim Magnetometer XML Input Mapping

[mvacanti]

Proposal Description

• Enable the user to map JSBSim variables to the bridge input properties via airframe configuration files as per previous.
• Provide an example model of varying magnetometer bias based on throttle input in px4_default_mag_sensor.xml.
• Does not modify the default behavior of any airframe
• NOTE: Stable JSBSim magnetometer startup behavior dependent on https://github.com/JSBSim-Team/jsbsim/issues/332

To Test

• Build separate branch of JSBSim: https://github.com/mvacanti/jsbsim/tree/pr-update-mag-startup
• Modify configs/quadrotor.xml to map desired variables
• Modify models/quadrotor.xml to include default_px4_mag_sensor.xml system file

[mvacanti]

I am done with my work, are you OK with the way I handled the open issue with JSBSim?

[Jaeyoung-Lim]

@mvacanti Yes, but does this mean that it would fail for other jsbsim distributions?

[mvacanti]

@Jaeyoung-Lim on the current JSBSim release it does not fail outright when using the magnetometer system file, for the first several seconds it creates an EKF magnetometer error that prevents arming due to erroneous startup data followed by a step-change. Once that clears it works fine. I have a PR for JSBSim that will be rolled into the next bug-fix release. The default behavior (no magnetometer JSB system) is unaffected by either.

[Jaeyoung-Lim]

@mvacanti Thanks, looks like your fix went into jsbsim! merging!