create GUI instance rather than CatalogGUI

[AlbertDeFusco]

This addresses #714 . The CatalogGUI instance does not work well if there are nested catalogs. Here's an example catalog.yaml file

name: my_local_catalog
sources:
    df:
      driver: csv
      args:
        urlpath: https://bit.ly/autompg-csv
    weather:
        driver: yaml_file_cat
        args:
            path: https://gist.githubusercontent.com/AlbertDeFusco/60a3d2031f89f1fed1d396bb367fe168/raw/401b29419608abd51985f45a81308bc3f4534410/weather.yaml

I've included a short screen capture to demonstrate what this change enables. You'll notice df in the list of sources in the root of the catalog and the nested weather catalog.

[martindurant]

https://intake.readthedocs.io/en/latest/gui.html doesn't mention the old version of this, which didn't work for you - so good!

[sophiamyang]

Hi @AlbertDeFusco, @martindurant mentioned that the GUI should look differently for intake 0.6.8? I don't think I see the updated GUI yet : )