examples/multi-layer/plane_wave plots

[rjleveque]

@mandli:

examples/multi-layer/plane_wave plots/setplot.py contains a lot of plt.hold statements that generate warnings that this is deprecated. Can they be removed?

More importantly, when I run this example now I get plots like this, very different from what's in the gallery at http://www.clawpack.org/gallery/_static/geoclaw/examples/multi-layer/plane_wave/_plots/frame0003fig0.html But the gauges look ok, so not sure what's up.

[mandli]

@hudaquresh just showed me this too for the storm surge code. I think it's because of the hold commands being used.

[mandli]

Working on fixes for this in #271.