Make graph auto nodes default where sensible

[hollasch]

Currently, the nodes specification may take the value auto, indicating that the set of nodes should be implicitly derived from the edge specifications.

Consider making this the default behavior, such that a graph's nodes will be the union of all nodes defined both implicitly (via edge references) or explicitly (in the nodes definition). Nodes defined implicitly will get a data value of null (()).

This may mean that some structures like node arrays may need to robustly handle sparse arrays, so we don't blow up allocation.

[hollasch]

It just occurred to me that if you didn't make auto nodes the default, you could replace the current (and only) LSON keyword auto with a null element for the nodes. Failing that, an empty array.

[hollasch]

Didn't realize I'd already thought of this. More convinced than ever to do this. If we do not automatically handle implicitly-defined nodes, then all parsers would need to handle the error of using a node that had not been declared. I don't see much advantage to requiring this.