Taking advantage of ROS header to store timestamps and frame_id

[adeguet1]

Whenever we use a ROS message with a header there is a timestamp and a frame_id. We should take advantage of this both when receiving from ROS and when sending to ROS.

In general, it would be good to see if the VTK MRML nodes already has some kind of timestamp. If they do, we should provide some conversion methods.

In subscribers, the callback could preserve both the timestamp and the frame_id along the last message. We can then add methods to retrieve these, something like GetLastTimestamp and GetLastFrameId. I implemented something similar for cisst-ros, see https://github.com/jhu-cisst/cisst-ros/blob/devel/cisst_ros_bridge/include/cisst_ros_bridge/mtsROSToCISST.h#L72.

In publisher, if the VTK data we're sending has a timestamp, we should use it and convert to ROS. If it doesn't, we should use the current ROS time.

[adeguet1]

It seems VTK objects all have an object name, see https://vtk.org/doc/nightly/html/classvtkObject.html#a596da3ed5dfb4179182c7868bf915178