structure containing edit information passed to
UF_CURVE_edit_move_mult_points.
 
explanation:  start segment (SS) and end segment (ES)
remain fixed while the inner segment (IS) is moved by
defining the new location of its end points (e1,e2)
 
 SS         e1      IS            e2                         ES
.....+......+.......................+.....................+................curve S(u)
     |      S(disp_par1)       S(disp_par2)  |
     |                                                        |
     S(lim_par1)                                     S(lim_par2)
 
methods to define direction/distance for moving ei:
 
displace_methodj  direction           distance
j=1               curve normal at ei  distancei
j=2               vectori             distancei
j=3               new location defined by dir_pti