Add CI

[clelange]

• Add small ROOT file with histograms that can be used for plotting
• Create script to use this ROOT file to create plots from
• Use this script in CI (GitLab CI or GitHub Actions or both)

[andrzejnovak]

Better to depend on skhep_data for test instead of a committed ROOT file eghttps://github.com/scikit-hep/mplhep/blob/master/tests/test_inputs.py#L41

Can also just create TH1s on the fly with uproot ROOT :).

[clelange]

Thanks for the pointer, but I'm not sure, it adds a dependency that might make it more difficult for people to contribute. Would surely need "contributing" docs.

[andrzejnovak]

I'm happy to help with image comparison tests in CI also. I've set them up for mplhep, but perhaps we set these up after we settle all the style tweaks.