Xtrack version 0.4.0

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