Incompatibility with ipywidgets 7.0.0

[slongwell]

gui.py
ipywidgets 6.0.0 works fine. Not sure if because 7.0.0 is not yet stable.

'Custom' properties added to widgets (Buttons, ToggleButtons, etc) seem to be ignored in 7.0.0.

Eg, with:

z_minus = widgets.Button(
            ax='z',
            sign=-1,
            button_style='primary',
            icon='fa-arrow-up')

ax and sign cannot be accessed as z_minus.ax and z_minus.sign (gives 'has no attribute' error).

For now, will enforce ipywidgets==6.0.0 in setup.py and requirements_dev.txt.