Save benchmark results in asv format

[riccardofelluga]

What does this PR do?

After discussing around some ways to implement a partial fix for #183 and #224, this option seems like a good compromise for the moment. Saving the results in this format allows for the benchmarks to be visually compared using the Airspeed velocity framework.

I hardcoded the output path but let me know if it's better to make it depend on an input flag.