New class and code for part 4

include/p_controller.h

@@ -0,0 +1,15 @@
+#ifndef P_CONTROLLER_H
+#define P_CONTROLLER_H
+
+class P_controller
+{
+private:
+    double Kp;
+
+public:
+    P_controller(double Kp);
+
+    double update(double ref, double actual);
+};
+
+#endif