Add helper functions to GraphMLReader/Writer to use filenames directly

At the moment for each graph one has to write:

```scala
val fos = new FileOutputStream(filename)
GraphMLWriter.outputGraph(graph, fos)
fos.close()
```

Including this change would reduce duplication und minify the code to

```scala
GraphMLWriter.outputGraph(graph, filename)
```

I think this is a common task many do ;)