recreate DEM comparison notebook

The IS2_DEM_comparison_WIP.ipynb notebook is still in the repo, but is not shown on the docs page because it is too outdated. icepyx now has some of the functionalities (e.g. data read in) that previously relied on other 2019 hackweek libraries, and the DEM used in the comparison cannot be shared publicly (though it is freely available to download). We need to migrate this example to a more easily programmatically accessible DEM and update it (including to match headings in other examples).