Node.producers() returns production() not actual producers

[mrvisscher]

Right now calling node.producers() will return the same Exchanges as node.products(). This seems incorrect for something like a product-type node where you would expect the result to be the production exchanges flowing from the product node. Meaning, the Exchanges object needs to be instantiated with reverse=True.

Right now there seems no way to get to the producer of a product other than instantiating an Exchanges object yourself like so:

Exchanges(prod.key, kinds=labels.technosphere_positive_edge_types, reverse=True)

Let me know if you want me to make a PR