More expressive filenames for graph-visualization

[jorgensd]

Currently the graph-visualisation keeps on overwriting the same file, F.pdf and S.pdf, which makes it tedious to visualize graphs for actual problems, where we have both a bilinear and linear form.

There are a few ways we could go about this, as the visualization happens in the IR.

1. Use the form_name as part of the output
2. Add explicit filename options.

[jorgensd]

The default format (pdf) should probably also be optional, as one could store the graphs as png or dot format as well.