ConstF Element

[ax3l]

In #3565, @dpgrote adds a simple plasma lens element.

plasmalens for a field modeling a plasma lens
This applies a radially directed plasma lens field that is uniform within the z extent of the element with
a sharp cut off at the ends.
This uses residence corrections, with the field scaled by the amount of time within the element for particles entering
or leaving it, to increase the accuracy.

This is a bit less general than the ConstF element in ImpactX: https://impactx.readthedocs.io/en/latest/usage/parameters.html#lattice-elements

constf for a constant focusing element. This requires these additional parameters:
<element_name>.ds (float, in meters) the segment length
<element_name>.kx (float, in 1/meters) the horizontal focusing strength
<element_name>.ky (float, in 1/meters) the vertical focusing strength
<element_name>.kt (float, in 1/meters) the longitudinal focusing strength
<element_name>.nslice (integer) number of slices used for the application of space charge (default: 1)

To harmonize, we could replace plasmalens with ConstF.