Implement backend API endpoint for query 3

[GoonerBrian]

Implement backend work for the following query from Deliverable 2:

How has the ratio of the production vs yield of a specific crop changed over a period of time between 2 countries?

This query will focus on the data contained in the Country, Product, and Element entities, and will be connected through the Produces and Collection-method relations. The ratio between production and yield will be calculated at the time the query is run and stored in a view. The user will specify which 2 countries they want to compare and the time period. The user can use this query to help identify trends in production and yields rates that may be related or contrasted between 2 countries.