Add option to output commodity graphs as PNG

Another probably-out-of-scope idea I had was that maybe we could invoke the `dot` command automatically for the user to e.g. generate PNG files. We shouldn't make it a requirement that the user has `graphviz` installed, so we could either detect it or add an option like `--convert-to=png` or something.

_Originally posted by @alexdewar in https://github.com/EnergySystemsModellingLab/MUSE2/pull/904#pullrequestreview-3324251479_