Add external focusing gradient by using the same formalism as `StageHipace`. See https://wake-t.readthedocs.io/en/latest/api/beamline_elements/_autosummary/wake_t.beamline_elements.PlasmaStage.html
Add external focusing gradient by using the same formalism as
StageHipace. See https://wake-t.readthedocs.io/en/latest/api/beamline_elements/_autosummary/wake_t.beamline_elements.PlasmaStage.html