version 0.5: llcrnrlon,llcrnrlat,urcrnrlon,urcrnrlat are now optional
keyword paramaters in Basemap.__init__,
version 0.4.2: removed 'preserve_magnitude' keyword from transform_vector.
version 0.3: axes instance no longer an argument to any Basemap method.
(gca is called to obtain the current axes instance instead).