Overload the support functions for SSQ2

Because any overloaded functions must be the same as those in the declaration of the aggregate, you must overload ssq2_iter and ssq2_combine to extend the SSQ2 aggregate to the complex data type.

CREATE FUNCTION ssq2_iter (partial complex, c complex)
   RETURNING complex;
   RETURN ROW (
      (partial.real + c.real*c.real - c.imag*c.imag),
      (partial.imag + 2*c.real*c.imag) 
      )::complex;
END FUNCTION;

CREATE FUNCTION ssq2_combine(p1 complex, p2 complex)
   RETURNING complex;
   RETURN ROW(p1.real + p2.real, 
           p1.imag + p2.imag)::complex;
END FUNCTION;