Install ROS and other tools, dependency for xavier nx

jetson_xavier_nx_ros_install

Note: ROS melodic use opencv3.2 by default while the kit software came with opencv4.1.1 pre-installed. Follow this guide to uninstall that. the opencv3.2 stable will be installed tgt with desktop-full of ros-melodic
How to uninstall opencv 4.1.1 (https://forums.developer.nvidia.com/t/trouble-with-catkin-make-hector-slam/126470) last comment.

1. Install ROS (Melodic for Ubuntu 18.04 LTS) as shown here (http://wiki.ros.org/melodic/Installation/Ubuntu)
2. For convenience, add the setup script for ros into your .bashrc or .profile so that it naturally run at every new instance of shell
3. (Optional) Install python catkin tools (a higer level, out-of-source build system for catkin). It has similar concepts to using the native catkin-make tools with improvements
  link: https://catkin-tools.readthedocs.io/en/latest/
4. (Optional) When using realsense D435i cameras as visual sensor, follow this github repository to install it (https://github.com/IntelRealSense/realsense-ros).
5. (Optional - Continue of 4) To launch RGB-D camera node, make sure you install this special launch library as stated in this issue thread (https://github.com/IntelRealSense/realsense-ros/issues/308)
6. (Optional - For ROS Octomap stuffs) Follow this thread (https://answers.ros.org/question/248242/cmake-could-not-find-octomap_ros-when-run-catkin_make/)