Xtrack version 0.71.0

Changes:

• Remove xfail in MAD-NG tests.
• Introduce xtrack.json.dump(...) and xtrack.json.load(...).
• Cleaner error message on failure to renormalize eigenvectors in twiss.
• Propagate default_tol when cloning an optimizer object.
• Autogenerate tages for optimizer targets.
• Add environment-like methods in Multiline.
• Add more customization features in TwissPlot.

Full Changelog: v0.70.3...v0.71.0