Feature: SML power meters: reset SML decoder

implement a function which allows to reset the SML decoder. this new
function is used after a datagram ends. for the SML HTTP power meter
this is simple: after all bytes from the request's answer have been
decoded, we reset the decoder. for the SML serial power meter, we
perform the reset after a datagram ended based on timing (no new bytes
have been received for a specific amount of time).

include/PowerMeterSml.h

@@ -20,6 +20,7 @@ protected:
     explicit PowerMeterSml(char const* user)
         : _user(user) { }
 
+    void reset();
     void processSmlByte(uint8_t byte);
 
 private:

lib/SMLParser/sml.cpp

@@ -189,6 +189,12 @@ void checkMagicByte(unsigned char &byte)
   }
 }
 
+void smlReset(void)
+{
+  len = 4; // expect start sequence
+  currentState = SML_START;
+}
+
 sml_states_t smlState(unsigned char currentByte)
 {
   unsigned char size;

lib/SMLParser/sml.h

@@ -92,6 +92,7 @@ typedef enum {
   SML_COUNT = 255
 } sml_units_t;
 
+void smlReset(void);
 sml_states_t smlState(unsigned char byte);
 bool smlOBISCheck(const unsigned char *obis);
 void smlOBISManufacturer(unsigned char *str, int maxSize);

src/PowerMeterHttpSml.cpp

@@ -107,5 +107,7 @@ String PowerMeterHttpSml::poll()
         processSmlByte(pStream->read());
     }
 
+    PowerMeterSml::reset();
+
     return "";
 }

src/PowerMeterSerialSml.cpp

@@ -110,6 +110,8 @@ void PowerMeterSerialSml::pollingLoop()
 
         lastAvailable = 0;
 
+        PowerMeterSml::reset();
+
         lock.lock();
     }
 }

src/PowerMeterSml.cpp

@@ -30,6 +30,12 @@ void PowerMeterSml::doMqttPublish() const
 #undef PUB
 }
 
+void PowerMeterSml::reset()
+{
+    smlReset();
+    _cache = { std::nullopt };
+}
+
 void PowerMeterSml::processSmlByte(uint8_t byte)
 {
     switch (smlState(byte)) {
@@ -52,12 +58,12 @@ void PowerMeterSml::processSmlByte(uint8_t byte)
         case SML_FINAL:
             gotUpdate();
             _values = _cache;
-            _cache = { std::nullopt };
+            reset();
             MessageOutput.printf("[%s] TotalPower: %5.2f\r\n",
                     _user.c_str(), getPowerTotal());
             break;
         case SML_CHECKSUM_ERROR:
-            _cache = { std::nullopt };
+            reset();
             MessageOutput.printf("[%s] checksum verification failed\r\n",
                     _user.c_str());
             break;