Map view improvements

This PR has some aesthetic and practical improvements for the map view.  These include fixing the x and y scale so that the map projection view is correct (#22), filling the land and water with a solid color (#24), removing axis labels and plot title, and filling the map pane with the view.

It also updates the navigation lines to use `interpolated_easting` and `interpolated_northing` (#23), though some bad points seem to still be present.

Known issue: islands are colored the same as water, because the polygon plot is unaware of the shapefile topology.