Xtrack

**Changes:**
- Introduce e-lens with arbitrary radial profile.
- Subtract arbitrary dipolar kick in wire beam element.
- Introduce LongitudinalLimitRect beam element.
- Make effective quantum radiation robust against dynamic beta.
- Element-by-element monitor now saves value at the end of the line.
- Tracker.twiss computes values at the end of the line (solves issues with tune computation when the last element is a drift). Added input flag values_at_element_exit to get optionally get values at the exit of the elements.
- More efficient implementation of Line.copy.
- Fixed issue with repeated names in Line.to_dict.
- Option inplace=False is not anymore allowed in Line.remove_inactive_multipoles, LIne.remove_zero_length_drifts, Line.merge_consecutive_drifts and Line.merge_consecutive_multipoles. User should explicitly make a copy of the line before the simplification steps.
- Further development of pipelined multibunch simulation capabilities, in view of strong-strong beam beam simulations.

**Changes:**
- Rewritten mad_loader, new features:
- Improved import of MAD-X expressions
- Import of tilt and offsets for elements and apertures from MAD-X elements and error tables
- Introduced Line.to_pandas and Line.check_aperture methods
- Introduced flag to skip loss location refinement for certain element classes, e.g. collimators

**Changes:**
- Introduced mechanism to pass custom headers to Tracker
- Introduced compilation flag to disable radiation in Multipole at compile time (beneficial for performance)

**Changes:**
- Refactored BeamElement class and metaclass to remove duplicated code and features from xobjects.HybridClass.
- Kernel declarations can be provided through the _kernel dictionary when subclassing BeamElement.
- Additional dependencies from xobjects types can be specified through the _depends_on list when subclassing BeamElement.
- Adapted to new Xobjects API (C source code provided in _extra_c_sources inside class declaration, _move_to renamed to move, XoStruct renamed to _XoStruct).
- Renamed per_particle_kernels to _per_particle_kernels.
- LocalParticle API is temporarily appended to the Particles class API just before compiling the track kernels. It is removed right after the compilation.

**Changes:**
- Removed dress_element function. Now there is only one mechanism to create beam elements which is to inherit from the BeamElement class.
- Across the package xobjects.DressedStruct has been replaced with xobjects.HybridClass following update in xobjects.
- Introduced pipeline features (preparation for strong-strong beam beam simulations):
- Collective elements can put the tracking on hold;
- Tracker handles the hold condition and can resume the tracking when the Tracker.resume method is called;
- Introduced PipelineMultitracker objects to handle multiple ongoing tracking sessions.
- Introduced custom per-particle kernels attached to beam elements.
- Regenerated all json files in test_data folder following refactoring of beam-beam classes in xfields.
- Introduced _arr2ctx internal method to transfer arrays to context if needed.
- extra_sources are now specified as member of the BeamElement class and not attached a posteriori to the XoStruct as done before.
- Adapted ducktrack to setup beam-beam elements from dictionaries produced with he new data structure implemented in xfields.

**Changes:**
- knl and ksl stored directly in Multipole and RFMultipole data structures (instead of bal). Improves significantly the speed when these are changed dynamically in the simulation.
- Restored set of canonically conjugate variables in R matrix, now using (x, px, y, py, zeta, pzeta)