Minor fixes to the Segmentation Primer Notebook

[denbonte]

As discussed, here's a brief list of the fixes to the notebook:

• If I got your intentions right, this cell should have a try-except and not an if-else (the cell fails to run and throws an error if result doesn't exist; I know that's not the case, but if a user were to run the whole notebook with "run all" this would break it!)
• I fixed the widgets, which are now working again (I changed the module name from viz to widgets in the bunch of commits I pushed before the release of pyplastimatch 0.4.3)
• I changed the coding style a bit for consistency 🙃

[denbonte]

I also removed the "Open to Colab" button at the top (GitHub now adds that automatically, so adding it in Colab makes it pop up twice when opened from GitHub - and you definitely don't need to see that button in Colab, since you're already in Colab).

Before:

After: