Fix differential signature

include/differential.h

@@ -7,14 +7,15 @@
 namespace numerics {
 
 template <typename argtype, typename valtype>
-class ScalarDifferential : public Function<argtype, valtype> {
+class ScalarDifferential
+    : public Function<Coordinate<argtype>, Coordinate<valtype>> {
   public:
     ScalarDifferential(Function<Vector<argtype>, valtype>& _func,
                        Differentiator<argtype, valtype>& _diff,
                        argtype _h = 1e-8)
         : func(_func), diff(_diff), h(_h) {}
 
-    Vector<valtype> operator()(numerics::Coordinate<argtype> x) const override {
+    Vector<valtype> operator()(Coordinate<argtype> x) const override {
         assert(x.dimension() == input_dimension());
         return diff(func, x, Vector<valtype>(x.dimension(), h));
     }
@@ -30,7 +31,7 @@ class ScalarDifferential : public Function<argtype, valtype> {
 };
 
 template <typename argtype, typename valtype>
-Function<Vector<argtype>, Vector<valtype>>
+ScalarDifferential<argtype, valtype>
 nabla(Function<Vector<argtype>, valtype>& func,
       Differentiator<argtype, valtype>& diff) {
     return ScalarDifferential<argtype, valtype>(func, diff);