Custom plots from a script

[stephane-caron]

Is your proposal related to a problem?

Arose from this discussion: #65

Describe the solution you'd like

Use qpsolvers_benchmark as a library rather than a command-line script so that users can more easily create their own custom plots.

Describe alternatives you've considered

Expanding the main script's CLI. Down that road we would end up having the whole matplotlib API surfacing in the CLI, which is not code we want to handle in the benchmark. Enabling users to more conveniently process benchmark results would be preferable.