% Implement binary operator for objects with a signal and a variance array.
%
% Generic method, generalised for sqw objects, that requires: 
%   (1) have methods to set, get and find size of signal and variance arrays:
%           >> sz = sigvar_size(obj)
%           >> w = sigvar(obj)          % w is sigvar object (has fields w.s, w.e)
%           >> obj = sigvar_set(obj,w)  % w is sigvar object
%   (2) have dimensions method that gives the dimensionality of the double array
%           >> nd = dimensions(obj)
%   (3) have private function that returns class name
%           >> name = classname     % no argument - gets called by its association with the class
%