topography plots

changes to Topography.plot and set mask=False by default when reading topo

@mandli, please take a look.  With these changes the quick start topo can be plotted nicely via:

```
    from clawpack.geoclaw import topotools
    import matplotlib.pyplot as plt
    plt.figure(figsize=(15,6))  # create figure with appropriate aspect ratio
    ax = plt.axes()

    npacific = topotools.Topography('npacific.tt3', topo_type=3)
    npacific.plot(axes=ax, limits=(-6000,3000))

    hawaii = topotools.Topography('hawaii.tt3', topo_type=3)
    hawaii.plot(axes=ax, limits=(-6000,3000), add_colorbar=False, plot_box='w')
    plt.axis(npacific.extent)
```