Generate data and control flow graphs by default

This migrates the data flow plotting from the graphviz module to pydot, as only the latter is provided by LCG, and adds control flow plotting, addressing #18 (closed), #48 (closed) (kinda), and #62 (closed).

We can't always generate graphics from the raw graphviz source as the necessary dot binary isn't provided by LCG. But dot is available on lxplus machines; we could consider checking to see if dot exists and converting to PDF in those cases, and (silently?) skipping graphics creation otherwise.