Add conversion from dot to png

[swerter]

Add the function to_png, which compiles the dot file into a png file.

[dunyakirkali]

This looks quite handy.
Any reason why the MR is still open?

[mindok]

@dunyakirkali - I'd imagine because it relies on an external command-line dependency (dot) which may or may not be installed. It would probably be better to have the function in the PR defined in your project rather than the library rather than have a library function fail due to a missing dependency that the library has no control over.

[bitwalker]

That's essentially the hang up. I think it is useful, and the PR is solid, but I'm hesitant to add a third party dependency, especially one needed for the test suite.

I think my preference would be to have a way to extend libgraph with capabilities like this in the form of third-party packages, as is done for many libraries. I just haven't decided how to handle that yet, and haven't had the time. I've left the PR open for those that wish to use it in their own projects by merging it into a fork, and until I can make a decision on how to move forward with it.

[dunyakirkali]

Makes sense