issue with GUI install

[jasmainak]

if we install hnn_core without gui option, you still have the hnn-gui command added and then it throws the error:

Traceback (most recent call last):
  File "/Users/mainak/anaconda3/envs/hnn/bin/hnn-gui", line 5, in <module>
    from hnn_core.gui.gui import launch
  File "/Users/mainak/anaconda3/envs/hnn/lib/python3.9/site-packages/hnn_core/gui/__init__.py", line 1, in <module>
    from .gui import HNNGUI, launch
  File "/Users/mainak/anaconda3/envs/hnn/lib/python3.9/site-packages/hnn_core/gui/gui.py", line 15, in <module>
    from ipywidgets import (HTML, Accordion, AppLayout, BoundedFloatText,
ModuleNotFoundError: No module named 'ipywidgets'

saving this for a rainy day ...