Formula for selling price of cow (based on buying price and health) should be factored out into its own class

[pconrad]

The formula for the selling price of a cow should be something that appears only once in the code base.

Here, it is buried in a controller method in
src/main/java/edu/ucsb/cs156/happiercows/controllers/UserCommonsController.java

It seems as if this would be better factored out into it's own class, and unit tested separately.

Screenshot

[pconrad]

Decided to move this into a code review for an earlier issue