Load, get_info and visualise rosbags

Closes #20 (closed)

• Added a fn to get topics and types from rosbags without having a ros distro
• Read rosbags and return data, containing the attributes earlier initialised
• Modified the visualisation fn for rosbags

Following development and testing to handle real data is adressed in further issues (#23 (closed) and in ros_datahandler) 😄