Create StructuralReform class

[anth-volk]

Fixes #328

This is a draft PR meant only for demonstration and testing purposes. A few high-priority potential features remain unimplemented due to the need for further discussion around them, including:

• Modifying variable metadata when using updating or neutralizing existing variables
• Loading relevant data from altered structural reform variables into the API metadata
• Refactoring how reforms are loaded in the -us package
• Testing