[BioC] Rgraphviz for Windows

[Jeff Gentry]

> An inelegant but probably functional workaround is to communicate with 
> graphviz through files. I don't know whether there is already  an export 
> function from "graph" objects in Bioconductor to .dot files, or whether 
> someone would like to contribute one?

Well, there's agwrite(), but iti s in Rgraphviz, which of course doesn't
exist for Windows. :)  But it will output a .dot file ...

I'm not very familiar, but I believe that there might be some mechanism of
graph->GXL->dot.