add the function to concatenate data when different fields msgs have

I added some lines to enable program to concatenate msg which has different fields.

From the first, I was to publish the sensor_msgs::PointCloud2 which has x,y,z,normal_x,normal_y,normal_z using pcl_ros' normal_3d_omp, but normal_3d_omp only publishes pcl::Normal.
Therefore, I was looking for concatenating pcl::XYZ and pcl::Normal nodelet class.
concatenate_data couldn't concatenate msg which has different fields, so I send this PR.