DipoleStepper with quadrupolar component for low momentum particles not correct

[Jochem Snuverink]

The DipoleStepper quadrupolar component implementation is not correct for low momentum secondaries and can result in an infinite loop.

The stepper should rather use the BDSQuadrupoleStepper for the quadrupolar part than its own implementation.

[Jochem Snuverink]

okay not so easy probably as dipole and quad step ideally done at same time

[Jochem Snuverink]

quadrupolar component has been temporarily disabled

[Jochem Snuverink]

unassigning myself, since no time to work on this issue anymore. This issue probably needs to be reviewed again once the fieldmap branch is finished.

[Laurie Nevay]

Will has implemented a new dipole quadrupole integrator that now correctly implements a dipole with quadrupolar component. Additional developments also provide the Lorentz beta needed for the transfer map as well as true Cartesian to curvilinear coordinate transforms. Using features from the dipole integrator, this tolerates low energy spinning particles.