Make system info sensors inherit from ValueProducer<T>

They have no use for being FileSystemSaveable.

src/sensesp/sensors/system_info.h

@@ -42,7 +42,7 @@ void connect_system_info_sensor(Sensor<T>* sensor, String prefix, String name) {
  * appears in your project's output. That is configured with
  * SensESPAppBuilder.
  **/
-class SystemHz : public FloatSensor {
+class SystemHz : public ValueProducer<float> {
  public:
   SystemHz() {
     elapsed_millis_ = 0;
@@ -53,10 +53,9 @@ class SystemHz : public FloatSensor {
   }
   String get_value_name() { return "systemhz"; }
 
- private:
+ protected:
   uint32_t tick_count_ = 0;
   elapsedMillis elapsed_millis_;
-  float system_hz_;
   void tick();
   void update();
 };
@@ -68,15 +67,15 @@ class SystemHz : public FloatSensor {
  * appears in your project's output. That is configured with
  * SensESPAppBuilder.
  **/
-class FreeMem : public IntSensor {
+class FreeMem : public ValueProducer<uint32_t> {
  public:
   FreeMem() {
     SensESPBaseApp::get_event_loop()->onRepeat(1000,
                                                [this]() { this->update(); });
   }
   String get_value_name() { return "freemem"; }
 
- private:
+ protected:
   void update();
 };
 
@@ -88,15 +87,15 @@ class FreeMem : public IntSensor {
  * appears in your project's output. That is configured with
  * SensESPAppBuilder.
  **/
-class Uptime : public FloatSensor {
+class Uptime : public ValueProducer<float> {
  public:
   Uptime() {
     SensESPBaseApp::get_event_loop()->onRepeat(1000,
                                                [this]() { this->update(); });
   }
   String get_value_name() { return "uptime"; }
 
- private:
+ protected:
   void update();
 };
 
@@ -108,15 +107,15 @@ class Uptime : public FloatSensor {
  * appears in your project's output. That is configured with
  * SensESPAppBuilder.
  **/
-class IPAddrDev : public StringSensor {
+class IPAddrDev : public ValueProducer<String> {
  public:
   IPAddrDev() {
     SensESPBaseApp::get_event_loop()->onRepeat(10000,
                                                [this]() { this->update(); });
   }
   String get_value_name() { return "ipaddr"; }
 
- private:
+ protected:
   void update();
 };
 
@@ -128,15 +127,15 @@ class IPAddrDev : public StringSensor {
  * appears in your project's output. That is configured with
  * SensESPAppBuilder.
  **/
-class WiFiSignal : public FloatSensor {
+class WiFiSignal : public ValueProducer<int> {
  public:
   WiFiSignal() {
     SensESPBaseApp::get_event_loop()->onRepeat(3000,
                                                [this]() { this->update(); });
   }
   String get_value_name() { return "wifisignal"; }
 
- private:
+ protected:
   void update();
 };