PathManager and databases

• opusapi.OPUS.get_between_times now has an optional target option to restrict the results to the intended target name.
• PathManager rewrite. The notebook repr now shows a list of found files.
• soliton wave through Saturn's rings now can be predicted and plotted with plotting.soliton_plot() (curtesy of Morgan Rehnberg's prediction polynom
• calib_ciss now logs INFO level progress messages and warns when one doesn't use map_projection=True
• tests now have an example notebook for io functionality
• io.print_db_stats() provides an overview of what data one has downloaded