Xtrack

**New features:**
- Introduced Tracker.cycle
- Introduced Tracker.twiss
- Introduced Tracker.filter_elements
- Configuration of space-charge lenses is done using Tracker.twiss instead of MAD-X
- Particles for footprint example are generated using xpart
- knl and ksl in Multipole and RFMultipole are linked to bal using xobject.LinkedArray

**Other:**
- Clean up examples for direct use in documentation
- Tests introduced for mad-x loader, energy ramp, ring with space charge (frozen, quasi-frozen and PIC)
- Added test_data with precisely matched HL-LHC lattice

**Fix:**
- Issue in Monitor.to_dict method

**New features:**
- Synchrotron radiation (average and stochastic) within Multipole
- Dipolar kicks in electron_lens

**New feature:**
- Tracker.find_closed_orbit works also on GPU

**Other modifications:**
- Slight modifications in example scripts adapting to modified interface of xpart.build_particles(...) and xpart.generate_matched_gaussian_bunch(...)

**Change in the Xsuite package structure:**
- The xline package is removed
- xline.Line becomes xtrack.Line
- instead of xline.Drift, xline.Multipole etc. use directly xt.Drift, xt.Multipole
- The Tracker objects are automatically updated when the line is modified (no need to regenerate the tracker)
- There is only one Particles class within Xsuite
- xline.Partices and xtrack.Particles replaced by xpart.Particles
- The reference python implementation of the main elements, which used to be in xline, is moved to a small package called ducktrack, installed together with xtrack, to be used only for testing purposes (not part of normal simulation workflows)
- Tracker(... sequence= ... ) becomes Tracker(... line= ... )

**New feature**
- Tracker class has new methods for closed-orbit search and one-turn-matrix calculation with finite differences

**New feature:**
- Aperture interpolation now works in the presence of thick and collective elements

**Interface modification:**
- Isolated elements do not update at_element and at_turn, which are instead handled only by the Tracker objects