Created an calibration for one sensor sample

examples/Calibration/Calibration.ino

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+/*
+  MQUnifiedsensor Library - calibrating an MQ2
+
+  Demonstrates the use a MQ2 sensor.
+  Library originally added 01 may 2019
+  by Miguel A Califa, Yersson Carrillo, Ghiordy Contreras, Mario Rodriguez
+ 
+  Added example
+  modified 27 May 2019
+  by Miguel Califa 
+
+ This example code is in the public domain.
+
+*/
+
+//Include the library
+#include <MQUnifiedsensor.h>
+
+//Definitions
+#define pin A0 //Analog input 0 of your arduino
+#define type 2 //MQ2
+
+//Declare Sensor
+
+MQUnifiedsensor MQ2(pin, type);
+unsigned long contador = 0;
+
+void setup() {
+  //Init serial port
+  Serial.begin(115200);
+  //init the sensor
+  MQ2.inicializar(); 
+  //Print in serial monitor
+  Serial.print("MQ2 - Calibracion");
+  Serial.print("Note - Make sure you are in a clean room and the sensor has pre-heated almost 4 hours");
+  Serial.print("Autonumeric, lecture");
+}
+
+void loop() {
+  //Read the sensor and print in serial port
+  int lecture =  MQ2.calibrate();
+  //Print in serial monitor
+  Serial.print(String(contador) + ",");
+  Serial.println(lecture);
+  //Increment counter
+  contador++;
+  //Wait to measure next sample
+  delay(400);
+}