Network specification and start hops modeling

[gilbertmike]

Semantics for network spec:

• Within a hierarchy, the fanout for a component should is still defined in the spatial of the component (or container). The network above the component is understood to be the one delivering to the array.
• When the network nodes attribute is used, then the nodes are understood to be "flat".

Note that the network has an attribute shape for defining the shape of the mesh, which is useful in flat-mode. We cannot use spatial since that means an array of current components, which corresponds to instantiating separate networks instead.

[gilbertmike]

Hops modeling isn't done yet. All diffs in model code is just preparing for the actual modeling code and cleaning up some modeling code.

The spec is done, however, and so I would like to pull into main at this state. There is one change in the Network spec since @tanner-andrulis last saw it:

• The Network node itself is no longer a Flat node. Instead, a Flat node should be created under the Network node if flatness is desired. This is cleaner because a component is still always a Leaf, and "structural" information is always in an explicit Hierarchical, Flat, or Fork.