Xtrack

**Changes:**
- Fixes in Fringe fields and expanded bend
- Fix in `Line.optimize_for_tracking`

**Full Changelog**: https://github.com/xsuite/xtrack/compare/v0.38.1...v0.38.2

**Changes:**
- `ShiftZeta` element shifts only time variable.

**Changes:**
- Introduce non-linear fringe effects in DipoleEdge element (can be enabled using `Line.configure_bend_model(edge='full')`)
- Introduce "compound elements" in Line.
- Elements within compounds can be grouped in twiss using the option `group_compound_elements=True`
- Rename `Bend.method` into `Bend. model`.

**Full Changelog**: https://github.com/xsuite/xtrack/compare/v0.37.2...v0.38.0

**Changes:**
- Prepare support for Bhabha scattering (soon to be deployed in Xfields)
- Fix issue with optimize_for_tracking when element internal records are active

**Changes:**
- Introduce the following thick elements: `Bend`, `Quadrupole`, `CombinedFunctionMagnet`.
- Introduce `Line.slice_thick_elements` method, equivalent to MAD-X makethin.
- Introduce time-dependent functions within xdeps variables, which are changed automatically during tracking.
- Introduce new method `Line.check_compatibility_with_prebuilt_kernels`

**Full Changelog**: https://github.com/xsuite/xtrack/compare/v0.36.5...v0.37.0

**Changes:**
- Docstrings update