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.