Adding outer aperture to elements
Currently OPAL-T only knows one aperture: if a particle is outside this aperture then it doesn't see the effect of the element, but it isn't stopped. We can't just kill all particles that are "outside" the element since the elements are placed in 3D space (consider circular lattices of e.g. ERLs). Solution an outer aperture. Kill the particle if it is inside the outer but outside the inner aperture. Use 10 * beta * c * dt as default for thickness of outer aperture.