Add comparison psy-view vs ncview to docs

By @StefanHagemann (see https://github.com/psyplot/psyplot/commit/0580fc8c69d1218f10839d5b9ce862c946d95d3c#r38976054)

I would add a statement about advantages of psy-view compared to ncview (It comprises (almost?) the whole functionality of ncview, but can do (much) more. A short comparison of the most important features can be found here --> make subpage with table that compares most important functionalities - What they can do both, only psy-view (ICON, export directly into pdf, jpg, png etc., much better resolution, better plot shaping options such as title, etc...), only ncview (very fast). With respect to a viewer this may be what people working with netcdf data want to know.