Topotools fix and Kahului harbor plot

I found a few more bugs in topotools and added an example of plotting real data -- cropped 1 second topo of Kahului Harbor.

Before merging: Perhaps remove references to Topography.z and Topography._z since I don't think 1d version of topography makes sense (whereas 1d versions of x and y do since X,Y are tensor product grids). @mandli: do you agree?