Convert node- and product-related lists to sets

[LarrySnyder]

e.g., raw_materials_by_product() etc should return sets, for speed

[LarrySnyder]

Also node predecessors, etc.

[LarrySnyder]

This will be more involved than I first thought. We can't take sets of SupplyChainNode or other objects, because sets can only contain immutable objects. So I think this will require (using _predecessors as an example):

• rename _predecessors to _predecessor_indices and make it a set of indices, not objects
• change predecessor_indices property so it draws directly from _predecessor_indices -- it will return a set
• change predecessors property so it obtains objects from indices, i.e., using SupplyChainNetwork.get_node_from_index() -- it will return a list
• look through code to err on the side of using predecessor_indices, not predecessors, whenever possible

See https://eng.lyft.com/hashing-and-equality-in-python-2ea8c738fb9d for a nice summary of the trouble with using custom objects in sets. (The approach I'm considering above doesn't convert them to immutables, though, it just uses indices instead of objects in the sets.)

[LarrySnyder]

This is not really a good idea. Lists are faster to iterate over than sets. Sets are faster to check membership but that's not the primary use case.