Pencil is slow: speed up

[freddieknets]

This is due to the implementation in xpart: one twiss and two times build_particles. Can this be sped up?

[freddieknets]

Currently, generate_pencil_on_collimator calls two twiss and five build_particles in total (two jaws)...

[freddieknets]

We could generate the pencil analytically, as we know the equations of the inner and outer ellipses, given the twiss parameters