Add concept of products to object model

[LeonChambers]

The object model has a relatively stable way of describing the software running on the system, but it has no sense of what plants are being grown in the system at a given point in time. We can define aggregate data points over an environment, but it would be nice to be able to record data at a plant-level. This would require a new object type representing an individual plant and a way to store data points directly related to those plants.