Add generic IsOff

Switch.cpp

@@ -3,3 +3,9 @@
 Switch::Switch() { this->type = Thing::SwitchType; }
 
 bool Switch::IsOn() { return false; }
+
+bool Switch::IsOff() {
+  bool isOn = IsOn();
+  bool isOff = !isOn;
+  return isOff;
+}

Switch.h

@@ -14,6 +14,8 @@ public:
   /// @brief Test of the switch is on
   /// @return true when the switch returns a HIGH value
   virtual bool IsOn();
+
+  virtual bool IsOff();
 };
 
 } // namespace RoboidControl