Overhaul of the plotting capabilities (now in dedicated module)!

fielddata and phasemap: Now use the new plottools module, cleanup of arguments
                        and documentation.
plottools: New module for pretty plotting and different annotations like text,
           colorwheels and colorbars.
colors: Colorbar now has a black circle outline and arrows pointing in cardinal
        directions.
diagnostics: Small documentation fixes and naming 'avg' --> 'avrg'.
docs: Renewed documentation.