fragile dependence on ipywidgets/bokeh versions

[cleemesser]

I noticed that as I updated to more recent versions of bokeh, the jupyter widgets (ipywidgets) stopped displaying.
Updating to the most recent ones with
conda install -c conda-forge ipywidgets

fixed this up -- as the widgets were not displaying even when using on its own.

this installs ipywidgets 7.1.0
and
widgetsnbextension-3.1.0 (which installs the javascript side into jupyter)
this may be the problem with the prior versions.

I may need to add this to requirements.txt