add app and update erddap notebook

[ocefpaf]

@lsetiawan can you take a quick look and merge?

[lsetiawan]

This looks great. How do I turn on the app extension? I need to probably add that to the dockerfile setup. Thanks.

[emiliom]

I'm curious: what's the "app extension"? What does it do?

[lsetiawan]

To allow the jupyter notebook extensions to work, there is some hook. For example,

jupyter nbextension enable     --sys-prefix --py ipywidgets