Xtrack

**Changes:**
- Line.insert_element allows replacement of aperture elements.
- Introduced getitem capability for Line providing object associated to a given index or name.
- Thresholds for checking one-turn matrix validity can now be modified at runtime.
- Import of MAD-X sequence can handle repeated elements.
- More robust algorithm for closed-orbit search, exception raised if solution is not found.
- Faster algorithm for marker insertion in Tracker.twiss(at_s=...)
- Fix in introduction of multipole errors in the presence of deferred expressions.

Fixes in Line.merge_consecutive_drift and Line.merge_consecutive_multipoles

**Fixes:**
- MAD-X expressions referring to tables are now skipped
- Error in expression conversion for RFMultipole.lag

**New features:**
- Introduced Wire beam element
- Line.insert_element(...) can insert a thin or think element at an arbitrary location `s`
- Tracker.twiss(...) can provide closed orbit, twiss parameters and phase advances at arbitrary `s` locations
- Tracker.track(...) can track particles from any element in the ring both in collective and non-collective modes. The element at which tracking starts is obtained from Particles.start_tracking_at_element if set.
- Introduced Line.get_s_position(...) method to get the longitudinal position of an element or a list of elements.

**Fixes:**
- "ecollimator" element is recognised by the MAD-X loader.
- Additional mathematical functions can be used in expressions (e.g. sinc).

**Changes:**
- Synchrotron radiation:
- Corrected dependence on energy deviation in average and quantum modes.
- Checked damping times and equilibrium emit against the emit module of MAD-X .
- Added optional computation of damping times and energy loss in Tracker.twiss.
- Synchrotron radiation modelling can be configured for the entire line using Line.configure_radiation(...).
- Random number generator is automatically initialised by the tracker.track(particles) if radiation is activated through Line.configure_radiation(...) .
- Changed sign convention for cavity voltage if reference particle has negative charge.
- Added example and tests for CLIC Damping ring.
- Disabled automatic symplectification in Tracker.twiss (found to degrade accuracy on vertical dispersion).
- Tracker.twiss now returns additional data:
- Scalars: T_rev, circumference, betz0.
- Along the beam line: delta, psigma and zeta coordinates of the 6D closed orbit.
- Optionally: damping_constants, partition_numbers and energy loss from radiation (see above).
- Tuned closed-orbit search algorithm. Info on numpy.fsolve convergence attached to returned particle object.
- Tracker.twiss checks that one-turn matrix is well defined
- Names of keyword arguments passed to beam element constructors are now checked (an exception is raised for unexpected names, e.g. typos).
- Tracker verifies that elements containing references to external data are allocated in the same buffer as the tracker.
- Fix in LossLocationRefinement (correct number of interpolated apertures).
- Simplified import of MAD-X expressions. Corrected management of expression in the presence of errors.
- Updated all examples.