Add window_stacking to ros_datahandler

Goal: allow direct stacking of the last (window_size) measurements in ros_datahandler. Was previously stacked ad-hoc in application code.

• remove dataset as attribute, that can just be done localy in loadbags
• optional argument to datahandler init to specify window_size
• handle case when window_size=1
• deque for ringbuffer
• tests