Add ability to plot topography using `contourf`

[mandli]

Currently the Topography class plot function only knows how to plot using pcolor or imshow. We should allow for plotting using contourf, maybe even automatically if the topography data is large enough.

[rjleveque]

There's a simple example of how to plot with countourf in the notebook http://nbviewer.ipython.org/url/clawpack.github.io/notebooks/topotools_examples.ipynb

We could allow lots more options, but I'm not sure how much we want to try to guess what users might do. I see the Topography.plot function as a simple way to see something but in general it's easy enough to plot directly what one wants.

But I have no objection to adding this functionality if someone wants to do so.

[mandli]

I was not thinking of anything to complex. Mostly I use the plot function as a quick way to view topography data anyway so putting too much complexity in there does not seem worthwhile.