TRACE 3-D code improvements

TRACE 3-D is an interactive beam-transport code for bunched beams that includes accelerating elements and linear space-charge forces. It has been integrated with an improved GUI (graphic user interface) based on the Shell for Particle Accelerator Related Codes. Recent modifications to the code include centroid tracking and an improved beam description consisting of a set of beam slices, each having its own 6D centroid and sigma matrix. This allows us to study some nonlinear effects, such as wakefields, that are related to the variation of the beam bunch along the longitudinal direction.