Adding new parameter yield

[Vismayak]

• Modify front end add new parameter for yield
• Rerun scripts to calculate and add yield to JSON

From Greg's suggestion
Add a new set of graphs for nutrient yield (load/drainage area), or replace the load graphs with yield graphs. Large watersheds generally have large loads simply because they are large. Dividing the load by drainage area (yield) quantifies the intensity of the loads that can be compared across watersheds of different size.