Component at Bus Lookup Lists

[ccoffrin]

Add data model helper functions that make it easy to compute the lookup lists for components at a specific bus (e.g. bus_gens). Use these helper functions in core_ref!.

[ccoffrin]

It is not immediately clear this could be re-used in core_ref!. A key question is if the data model version of this should include inactive components or not. Initial thought is that it should include all components form the data model.