Xtrack version 0.28.0

Changes:

• Methods and attributes of the Tracker class transferred to the Line class. As of this version the user should never need to access the Tracker object directly but can perform all operations using the Line object (e.g. line.track(...), line.twiss(...), line.configure_radiation(...), line.compensate_energy_loss(...), etc.)
• Introduce Line.to_json and Line.from_json methods.
• Introduce replace_in_expr option in mad_loader to replace a set of strings in all imported expressions.
• Introduce expressions_for_element_types in mad_loader to import deferred expressions only for elements of selected types (e.g. only for kicker elements).
• Introduce Line.closed_orbit_correction.
• Introduce Multiline class to handle multiple beam lines sharing a set of knobs.
• In Line.twiss add possibility to have multipole strengths in the output table.
• In Line.match introduce possibility to have targets quantities at specific locations in the beam line.
• In Line.match, in the Jacobian calculation, introduce support for steps of different size for the different variables.
• In Line.match introduce possibility to have different scales for the different targets.
• Fix in mad_loader in the import of Translation elements.