- GPS and battery were publishing at > 800Hz

 - update to latest sitl_gazebo

thread for offboard publishers, add asserts for topics to come up (simulation ready) and set mode and arming, use home_position topic as better indicator of when the simulation is ready, add more feedback to rosinfo, make timeouts meaningful (in seconds), add land and extended state values

This is important to have the probation times set up correctly and to silence land detected messages for systems that are not actually flying and just on the bench.

The goal is to force developers to use the correct target with the correct flash size. This prevents criticial functionality missing and is in particular important for FMUv2/FMUv3 boards. It is unmaintainable otherwise for the Pixhawk series.