Move geometrical aspects of the Beam from the Beam to the BeamAttenuator class

[Mateasek]

I think it would be better if the geometrical description of the beam was moved from the Beam class to the BeamAttenuator class. It is the BeamAttenuator which provides the density profile which drives the beam shape. In future it will be easier to cover accurately different attenuators.

For now this would mean moving geometrical properties length, divergences and sigma and _generate_geometry and _configure_geometry to the SingleRayAttenuator.

Could I ask @CnlPepper, who is the author of the current code, for an opinion about this step?

[Mateasek]

This would also fix some of the issues discussed in #286