Relay should construct the topic message as a proper JSON string

[seanyen]

Ideally the topic messages received by relay should be a proper JSON string.

For example, if the subscribed topic is /acml_pose, and then the message can be:

"/amcl_pose": {
  "amcl_pose": {
    "header": {
      "seq": "placeholder",
      "stamp": "placeholder",
      "frame_id": "placeholder" 
    },
    "pose": {
      "pose": {
        "position": {
          "x": "placeholder",
          "y": "placeholder",
          "z": "placeholder"
        },
        "orientation": {
          "x": "placeholder",
          "y": "placeholder",
          "z": "placeholder",
          "w": "placeholder"
        }
      },
      "covariance": ["placeholder"]
    }
  }
}

Here are existing implementations which does the JSON conversion:

• https://github.com/uos/rospy_message_converter
• https://github.com/RobotWebTools/rosbridge_suite