- use `gazebo-classic` everywhere consistently referring to the original Gazebo (eg version 9,10,11)
- additional `gazebo_*` helper targets added for compatibility, but warn about deprecation and tell you the new target naming
- use `gz` everywhere when referring to Gazebo (aka Ignition Gazebo or new Gazebo)
- new modules/simulation directory to collect all simulators and related modules
- new Tools/simulation directory to collect and organize scattered simulation submodules, scripts, etc
- simulation module renamed to simulator_mavlink
- sih renamed to simulator_sih (not a great name, but I wanted to be clear it was a simulator)
- ignition_simulator renamed to simulator_ignition_bridge
- large sitl_target.cmake split by simulation option and in some cases pushed to appropriate modules
- sitl targets broken down to what's actually available (eg jmavsim only has 1 model and 1 world)
- new Gazebo consistently referred to as Ignition for now (probably the least confusing thing until we fully drop Gazebo classic support someday)
* Update launch files to support multi-video stream in SITL
* Pass extra args to jinja_gen.py inside gazebo_sitl_multiple_run.sh to enable multi-video streaming
* remove non-functional gazebo headless arg
* remove unused namespace args
* simplify mavros launch, use mavros's px4.launch instead
* fix single_vehicle_spawn spelling
* formatting with xmllint: remove empty lines, set intent to 4 spaces, add xml tag for editors, add comments to break up spections
* remove old and deprecated launch files