parse generic node and edge attributes?

[pazz]

• add EdgeAttr similar to node attributes
• parse anything of the form "attrname(value)" ?
• getter for nodes with given value for given attribute (final nodes?)

[pazz]

actually better to use petgraph and dot_parser to parse arbitrary dot graphs into a pegraph::Graph.

Minor difficulty: ensure that there is an "available" attribute (fail if not) and parse the included formula?