Example notebooks using policyengine package

[MaxGhenis]

Let's add notebooks covering:

• Parametric reform
• Structural reform adding a new variable
• Structural reform overwriting an existing variable
• Creating a population-level chart
• Creating a household-level chart

[MaxGhenis]

This is partly in https://policyengine.dev/analysis.html, now need to use the policyengine package instead of the openfisca package to extract summaries