add test for autodiff about forwarding func

test/forward/1single_variable_function.cpp

@@ -0,0 +1,25 @@
+// NOTE: 单变量函数的导数
+
+// C++ includes
+#include <iostream>
+
+// autodiff include
+#include <autodiff/forward/dual.hpp>
+using namespace autodiff;
+
+// The single-variable function for which derivatives are needed
+dual f(dual x)
+{
+    return 1 + x + x*x + 1/x + log(x);
+}
+
+int main()
+{
+    dual x = 2.0;                                 // the input variable x
+    dual u = f(x);                                // the output variable u
+
+    double dudx = derivative(f, wrt(x), at(x));   // evaluate the derivative du/dx
+
+    std::cout << "u = " << u << std::endl;        // print the evaluated output u
+    std::cout << "du/dx = " << dudx << std::endl; // print the evaluated derivative du/dx
+}