Tag for preprint

[tschm]

I have introduced a first tag for the version of the code used for the preprint published. I should mention this in the readme. We can have more tags for the paper and different versions of it. We can keep main moving a make sure it won't stop working.

[tschm]

@kasperjo @phschiele @stephenpboyd

[phschiele]

Good idea, makes it easy to see which state was used in the paper.