Renamed PortComment.binding field variable to PortComment.data and added
a binding() method that returns PortComment.label if not null, and
PortComment.data otherwise.
Workflow.Builder no longer builds channels for in ports with no
corresponding output ports.
Update test to reflect change in port field names.
Added classes implementing basic rendering of the workflow graph as a
dot file.
Added support to the command line interface for writing a dot file
representing the workflow graph.