Feature/end track marker+pyheadtail conversion

[martinschwinzerl]

Based on PR #50, this one adds an end-of-tracking marker element which would

• allow easier automatic setup of track_line from e.g. the MAD-X loader
• allow simplifications to the tracking logic once we implement the Single-Particle tracking optimisation for SixTrackLib

Additionally, it adds a stub element for integrating with PyHEADTAIL which also could be placed programmatically before and after the end-of-tracking marker and allow to call the conversion functions from/to PyHEADTAIL data model from a GPU kernel in SixTrackLib.

In its current form, the PR is not intended to be merged but as a basis for reviewing code and a central point for discussions.

[aoeftiger]

Currently, the conversion is done in an explicit manner via python (which is slow, see https://github.com/aoeftiger/PySixHead/blob/main/tracker.py#L85 ):
PyHEADTAIL Particles (x, x', y, y', zeta, delta) --> SixTrackLib Particles

def pyht_to_stl(self, beam):
        stl_particles = self.trackjob.particles_buffer.get_object(0)
        n = beam.macroparticlenumber

        stl_particles.x[:n] = beam.x
        stl_particles.px[:n] = beam.xp
        stl_particles.y[:n] = beam.y
        stl_particles.py[:n] = beam.yp
        stl_particles.zeta[:n] = beam.z
        stl_particles.delta[:n] = beam.dp

        stl_particles.particle_id[:n] = beam.id
        stl_particles.state[:n] = beam.state
        stl_particles.at_turn[:n] = beam.at_turn
        stl_particles.at_element[:n] = beam.at_element
        stl_particles.s[:n] = beam.s

        # further longitudinal coordinates of SixTrackLib
        rpp = 1. / (beam.dp + 1)
        stl_particles.rpp[:n] = rpp

        restmass = beam.mass * c**2
        restmass_sq = restmass**2
        E0 = np.sqrt((beam.p0 * c)**2 + restmass_sq)
        p = beam.p0 * (1 + beam.dp)
        E = self.mathlib.sqrt((p * c) * (p * c) + restmass_sq)
        psigma =  (E - E0) / (beam.beta * beam.p0 * c)
        stl_particles.psigma[:n] = psigma

        gamma = E / restmass
        beta = self.mathlib.sqrt(1 - 1. / (gamma * gamma))
        rvv = beta / beam.beta
        stl_particles.rvv[:n] = rvv

        self.trackjob.push_particles()

--> this functionality is what would be required, but I'm unsure how this could be integrated as the track function seems unappropriate..?