Update combining models notebook

[lymichelle21]

• Add table of contents
• Add labels to graphs
• Fix formatting for consistency

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[kjjarvis]

These improvements to notebook #6 look great. Here's just one question.

I see that this requires use of the dataset, so perhaps we should hold off on approval/merging until we fix the issue, just to be sure everything is running correctly. But great PR and ready to go as soon as we get the data fixed!

[kjjarvis]

Looks great!