Fix signature of CGstep

include/optimizer.h

@@ -72,7 +72,7 @@ class ConjugateGradient : public Optimizer<double, double> {
 
     Coordinate<double> CGstep(Function<Coordinate<double>, double>& func,
                                 Coordinate<double>& start,
-                                double precision) const override;
+                                double precision) const;
 
   protected:
     Differentiator<double, double>& diff;