Beam radius options.

This option was added initially for use in one of the options for changing the diameters of space-charge tubes. It is also used in the option at the end of the list of ‘contours’, to output the radius of a beam as a function of z.

The option is intended only for beams that have axial (cylindrical) symmetry about the z axis. Two definitions of the radius of the beam are available. In the first the mean square radius of the rays is calculated and is then used to give an outer radius equal to sqrt(2*mean_square_radius), which is exact for a uniformly populated circle. In the second the radius of the outer ray is simply used.