Add option to specify named blocks of nodes for easily adding many edges at once

[macks22]

For instance, in a Markov model, we have a sequence of nodes, each with an arrow to the next. Then if we have a common parameter across those nodes, we'll want to draw an edge from it to each node in the sequence. Here, specifying the sequence of nodes as a node block would then allow Node.with_edges_to to specify edges to all nodes in the sequence at once.