Xfields

**Fix:**
- Fixes in tests

**Fix:**
- Adapt to new xtrack.Line data structure

**New feature:**
- Introduced tool to setup space-charge interactions in a given line (beam sizes obtained through xtrack.Tracker.twiss)

**Bugfix**
- Problem found in Spacecharge.copy()

**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 to be 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 tracking workflow)

**Bugfix**
- added missing property in SpaceCharge3D