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)

Tools/simulation/sitl_multiple_run.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+# run multiple instances of the 'px4' binary, but w/o starting the simulator.
+# It assumes px4 is already built, with 'make px4_sitl_default'
+
+# The simulator is expected to send to TCP port 4560+i for i in [0, N-1]
+# For example jmavsim can be run like this:
+#./Tools/simulation/jmavsim/jmavsim_run.sh -p 4561 -l
+
+sitl_num=2
+[ -n "$1" ] && sitl_num="$1"
+
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+src_path="$SCRIPT_DIR/.."
+
+build_path=${src_path}/build/px4_sitl_default
+
+echo "killing running instances"
+pkill -x px4 || true
+
+sleep 1
+
+export PX4_SIM_MODEL=iris
+
+n=0
+while [ $n -lt $sitl_num ]; do
+	working_dir="$build_path/instance_$n"
+	[ ! -d "$working_dir" ] && mkdir -p "$working_dir"
+
+	pushd "$working_dir" &>/dev/null
+	echo "starting instance $n in $(pwd)"
+	$build_path/bin/px4 -i $n -d "$build_path/etc" >out.log 2>err.log &
+	popd &>/dev/null
+
+	n=$(($n + 1))
+done