simulation organization and cleanup

- 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)

launch/px4.launch

@@ -10,7 +10,6 @@
     <arg name="interactive" default="true"/>
 
     <env name="PX4_SIM_MODEL" value="$(arg vehicle)" />
-    <env name="PX4_ESTIMATOR" value="$(arg est)" />
     <arg unless="$(arg interactive)" name="px4_command_arg1" value="-d"/>
     <arg     if="$(arg interactive)" name="px4_command_arg1" value=""/>
     <node name="sitl_$(arg ID)" pkg="px4" type="px4" output="screen" args="$(find px4)/build/px4_sitl_default/etc -s etc/init.d-posix/rcS -i $(arg ID) $(arg px4_command_arg1)">