Update networkset logic at bottom of example notebook to handle case where networkset exists

[coleslaw481]

The logic at the bottom of the example notebook needs to handle case where a networkset with same name exists: https://github.com/ucsd-ccbb/NetColoc/blob/main/example_notebooks/ASD_CHD_NetColoc_analysis.ipynb

Easiest might be to just append a timestamp at end so we always create a new network set

[brinrosenthal]

timestamp appended