def relative_to_many(mode, *xs)
      xs.map{|x| relative_to(x)}.send(mode)
    end
def relative_to_many(mode, *xs)
      xs.map{|x| relative_to(x)}.send(mode)
    end
This method is redefined in derived classes to compute the tolerance value in relation to the values xs;
 mode must be either :max or :min, and determines if the largerst (relaxed condition) or smallest
(strict condition) of the relative tolerances is returned.

 If not redefined, relative_to will be used, but redefining this method can be used to optimize the
 performance