Save figures in a path relative to the script

[QuLogic]

This allows running them from the scripts directory, or the top of the checkout.

Also change plt.savefig to fig.savefig.

[jimustafa]

Maybe ROOT_DIR, or something similar, rather than REPO? Also, many of the plt.savefig to fig.savefig changes were made in #88, but that PR has languished for some time now; anyhow, the merge conflicts should not be too difficult to resolve.

[QuLogic]

Sure, used ROOT_DIR (to be honest, I was not really happy with REPO either, but couldn't think of what to use instead.)