Changes proposed in this pull request

This PR separates ReactorNet visualization tweaks from #1788 to facilitate further reviews.

• Add wall names to ReactorNet graphviz illustrations.
• Partially addresses review comment by removing diagram.view

There is a caveat here, - see Cantera/enhancements#212, - as well as per @speth's comment:

Regarding the extension of the graph idiom, I think there's a simplification that may be worth considering. Instead of seeing just Reactor and ReactorSurface objects as nodes in the graph, you could also see valves and flow devices as nodes, with the edges of the graph just being the abstract connections among all of the entities that make up the reactor network. This way, there's only the need for one new base class, and no need for anything to sit between a Reactor and a ReactorSurface. This also gets closer to the original intent of Cantera/enhancements#212, and may work better as the amount of information you might want to display about a connector increases, given some of the limitations of how Graphviz handles (or doesn't) text associated with edges. That said, I do like the Connector class as a generalization combining walls and flow devices, and there's probably some further room to adjust this idea.

On the other hand, there is @Naikless's original intent:

In classical graph terminology, when I introduced the visualization, I envisioned the flow devices and the walls as "edges" that each connect two "nodes" that are reactors or reservoirs. So now they are represented as arrows of different shape and color. This way, the direction of both material and energy flow can be indicated relatively easily. Their main property (the amount of flow) is still displayed.

Having them represented as nodes could imo quickly lead to very cluttered networks, especially since there are quite a few scenarios conceivable where two reactors have a multitude of connections to one another. But I am happy to hear your arguments on this.

This PR attempts to stick with the original intent.

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Closes Cantera/enhancements#212 (minor aspect of this PR)

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Checklist

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