Add a module for external functions.

functions.cpp

+// functions.cpp
+//
+// The Makefile expects that the project solution has main() in a file
+// named project*.cpp
+//
+// These functions are declared in functions.h and implemented in functions.cpp
+
+#include <iostream>
+using std::cout;
+using std::endl;
+
+#include "functions.h"
+
+
+void calculate(int x, int y) {
+    cout << x << " + " << y << " = " << x + y << endl;
+}

functions.h

+#ifndef FUNCTIONS_H
+#define FUNCTIONS_H
+
+void calculate(int x, int y);
+
+#endif