Centralizing Raster support and plotting, gu vs. utils.Raster

[dshean]

The plot_dem support in altimetry.py uses geoutils Raster class, while other modules use the custom Raster defined in utils.py. I'm guessing this is just evolving preference and best practices.

I think maybe some of the plot_atl06sr in altimetry.py should be a class, mirroring some of the centralized plotting in other modules.

This issue is mostly for high-level design discussion - learning as I go here, probably easiest to discuss before spending a lot of time on this.