Add beforeaxes attribute to ClawPlotAxes and run it in plot_frame

[rjleveque]

Useful to plot background image before each time frame plot, e.g. Google Earth image for GeoClaw.

For example, add something like this to the top of setplot.py:

image = plt.imread('image.png')

def background_image(current_data):
    from pylab import imshow
    extent = [x1,x2,y1,y2]
    imshow(image,extent=extent)

and then for some ClawPlotAxes setup:

    plotaxes.beforeaxes = background_image