added arguments classes and extend to vis.polar_plot
added arguments classes and extend to vis.polar_plot

<classes> is a sequence of class boundaries. Colors are assigned according to the
class boundaries and the colormap argument. It is recommended to use this plotting
mode because the behaviour of automatic stretching is quite unpredictable.

The argument <extend> determines the behaviour of the colorbar (see library reference)

modified one example accordingly.

wradlib/wradlib,heistermann/wradlib,kmuehlbauer/wradlib,heistermann/wradlib,kmuehlbauer/wradlib,wradlib/wradlib