replace return value to 'unsigned long' since all the functions return that type, not 'int' (type size and signedness mismatch)

BaseMQ.cpp

@@ -65,7 +65,7 @@ float BaseMQ::readRs() {
   return rs;
 }
 
-int readScaled(float a, float b) {
+unsigned long readScaled(float a, float b) {
   float ratio = readRatio();
   return exp((log(ratio)-b)/a);
 }

BaseMQ.h

@@ -31,7 +31,7 @@ protected:
 	uint8_t _pin;
 	uint8_t _pinHeater;
 	float readRs();
-	int readScaled(float a, float b);
+	unsigned long readScaled(float a, float b);
 	float calculateResistance(int rawAdc);
 	virtual float getRoInCleanAir() const = 0;
 	virtual int getRL() const = 0;