[TODO] Add image_topic.h

[Corsair-cxs]

Hi, I'm trying to build ros_openvino_toolkit dev-ov 2020.3, but it failed. It seems that some functions and variables in image_topic.cpp is undeclared. May be we should add a file named image_topic.h in dynamic_vino_lib.

namespace Input {
/**
 * @class ImageTopic
 * @brief 
 */
    class ImageTopic {
    public:
        explicit ImageTopic(rclcpp::Node::SharedPtr node) : _node(node) { }
        bool initialize();
        bool initialize(size_t width, size_t height);
        void cb(const sensor_msgs::msg::Image::SharedPtr image_msg);
        bool read(cv::Mat * frame);

    private:
        //  those variable are undeclared
        node_;
        image_;
        image_count_;
        //image_count_.increaseCounter();
    }
}

[LewisLiuPub]

PR #92 is aimed to fix this issue.