Xtrack version 0.51.0

Changes:

• Second order dispersion and second order chromaticity added to twiss result.
• Add chromatic properties to allowed keyword arguments for optimization targets.
• Search for off-momentum closed orbit when computing chromatic properties in twiss.
• Closed orbit search performed with Jacobian optimizer instead of numpy fsolve.
• Possibility to use the optimizer on a generic callable.
• Implement solenoid backtrack.

Full Changelog: v0.50.1...v0.51.0