added clockstretch support for ESP8266 board

src/SparkFun_SCD30_Arduino_Library.cpp

@@ -40,6 +40,23 @@ boolean SCD30::begin(TwoWire &wirePort)
   //We expect caller to begin their I2C port, with the speed of their choice external to the library
   //But if they forget, we start the hardware here.
   _i2cPort->begin();
+
+  /* Especially during obtaining the ACK BIT after a byte sent the SCD30 is using clock stretching  (but NOT only there)!
+   * The need for clock stretching is described in the Sensirion_CO2_Sensors_SCD30_Interface_Description.pdf
+   *
+   * The default clock stretch (maximum wait time) on the ESP8266-library (2.4.2) is 230us which is set during _i2cPort->begin();
+   * In the current implementation of the ESP8266 I2C driver there is NO error message when this time expired, while
+   * the clock stretch is still happening, causing uncontrolled behaviour of the hardware combination.
+   *
+   * To set ClockStretchlimit() a check for ESP8266 boards has been added in the driver.
+   *
+   * With setting to 20000, we set a max timeout of 20mS (> 20x the maximum measured) basically disabling the time-out 
+   * and now wait for clock stretch to be controlled by the client.
+   */
+
+   #if defined(ARDUINO_ARCH_ESP32) || defined(ARDUINO_ARCH_ESP8266)
+   	_i2cPort->setClockStretchLimit(200000);
+   #endif
 
   //Check for device to respond correctly
   if(beginMeasuring() == true) //Start continuous measurements