title: How to merge 2 ROS topics into 1?
body: <p>I have 2 topics being published by an intel realsense camera. One topic is posting a depth image, and the other is posting a color image. Every color image has a corresponding depth image with the same timestamp in the header. I want to merge the color and depth topic into one topic that would publish the pairs of depth and color images. Is there a ROS function that does this based on timestamps?</p>
  
  <p>Here are the subscribers I have created:</p>
  
  <pre><code>self.image_sub = rospy.Subscriber("image", Image, mask_detect, queue_size=1, buff_size=2**24)
  depth_image_sub = rospy.Subscriber("depth_image", Image,
  aquire_depth_image, queue_size=1000)
  
  </code></pre>
  
  <p>I want to be able to do something like this (psuedocode):</p>
  
  <pre><code>color_depth = rospy.Subscriber(["image", "depth_image"], callback_function, mergePolicy="EXACTTIME")
  </code></pre>
  
  <p>Is there a standard way of doing this in ROS, or a simple way of doing it?</p>