Sequentially get the IR and RGB data from openni_node

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The openni driver does not allow streaming both ir and rgb at the same time, but there is a way to switch between ir mode and rgb mode. The driver looks at the number of subscribers on the rgb and ir topics, and based on the number of subscribers decides to enable ir mode or rgb mode. When you have one or more subscribers to the rgb topic, the driver will be in rgb mode, independent of the number of subscribers to the ir topic. When you have zero subscribers to the rgb topic and one or more subscribers to the ir topic, the driver will be in ir mode.

To implement this, you can use the ros::topcic::WaitForMessage method. This method will subscribe to a topic, receive one message, and then unsubscribe from the topic. So you can call this method sequentially on the rgb and the ir topic to switch between rgb/ir mode and receive rgb/ir images.