function s = group(si,pos,c)
% s = group(si,pos,c)
% Group of structures si with positions pos.
% si  :  Structure to be replicated
% pos :  There are three options for pos:
% pos = n*3-matrix of (x,y,z)-positions
% pos = n*2-matrix of (y,z)-positions
% pos = n*1-matrix of z-positions
% c   :  Vector of n complex values for phase 
%        shift of excitations of the n substructures
%        If c is scalar this value will be applied 
%        to all structures.
