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/gazebo/setup_gazebo.bash

@@ -0,0 +1,25 @@
+#!/bin/bash
+#
+# Setup environment to make PX4 visible to Gazebo.
+#
+# Note, this is not necessary if using a ROS catkin workspace with the px4
+# package as the paths are exported.
+#
+# License: according to LICENSE.md in the root directory of the PX4 Firmware repository
+
+if [ "$#" != 2 ]; then
+    echo -e "usage: source setup_gazebo.bash src_dir build_dir\n"
+    return 1
+fi
+
+SRC_DIR=$1
+BUILD_DIR=$2
+
+# setup Gazebo env and update package path
+export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:${BUILD_DIR}/build_gazebo
+export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:${SRC_DIR}/Tools/simulation/gazebo/sitl_gazebo/models
+export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${BUILD_DIR}/build_gazebo
+
+echo -e "GAZEBO_PLUGIN_PATH $GAZEBO_PLUGIN_PATH"
+echo -e "GAZEBO_MODEL_PATH $GAZEBO_MODEL_PATH"
+echo -e "LD_LIBRARY_PATH $LD_LIBRARY_PATH"