Fix missing return statements

src/optimizer.cpp

 #include "optimizer.h"
 
+using namespace numerics;
 /*
    For an explanation on what the three methods do see the exercise sheet, your
    lecture notes as well as the internet :)
@@ -7,14 +8,19 @@
 
 Coordinate<double>
 GradientDescent::optimize(Function<Coordinate<double>, double>& func,
-                          Coordinate<double>& start, double precision) const {}
+                          Coordinate<double>& start, double precision) const {
+    return start;
+}
 
 Coordinate<double>
 GradientRootfinder::optimize(Function<Coordinate<double>, double>& func,
                              Coordinate<double>& start,
-                             double precision) const {}
+                             double precision) const {
+    return start;
+}
 
 Coordinate<double>
 ConjugateGradient::optimize(Function<Coordinate<double>, double>& func,
                             Coordinate<double>& start, double precision) const {
+    return start;
 }