Xtrack

**New features:**
- Added BeamInteraction element to handle processes that produce secondary particles
- Removed num_particles attribute in from Particles class. Replaced with _capacity, _num_active_particles,_num_lost_particles. The last two are updated only on CPU while on gpu the information must be retrieved from Particles.state.
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Other clean up and fixes are also included.

**Fix:**
- Scale transverse momenta in LocalParticle_update_p0c

**Fix:**
- (zeta, delta) update in linear transfer matrix

**New features:**
- Electron lens element has been introduced (thanks phermes)
- Linear Transfer matrix element has been introduced (thanks xbuffat).
- PyHEADTAIL interface has been improved to support slicing features (thanks xbuffat)
- Multithreading via OpenMP is now fully supported (it can be enabled when creating the context, e.g. xo.ContextCpu(omp_num_threads=8). At the source code level the developer needs to use the annotation \\start_per_particle_block which introduces the necessary directives (see [documentation](https://xsuite.readthedocs.io/en/latest/newbeamelement.html)).
- Particles coordinate names in xtrack, xline and xpart have been aligned. Now it is possible to create xtrack particles form xpart or xline particles simply by xtrackpart = xt.Particles(**xpartpart.to_dict()). The function xt.pyparticles_to_xtrack_dict has been removed.