Network Node Ifc
Common abstraction over the node types that may participate in a ksl.modeling.supplychain.network.MultiEchelonNetwork's arborescent tree: today InventoryHoldingPoint (holds inventory) and InventoryCrossDock (routes demand through without holding inventory).
The network keys its topology, level tracking, and per-node demand-generator attachment on this interface so that a single carrier-wiring policy (ksl.modeling.supplychain.network.TransportStrategy) applies uniformly to both kinds of node.
Extends DemandFillerIfc (for the DemandFillerIfc.demandCarrier slot, the receive/fill API, and name/isAvailable from the inherited identity / availability interfaces) and DemandSenderIfc (for the DemandSenderIfc.demandFiller slot used to point at an upstream supplier).