Driver for Sensirion SHT3x sensor added

This is a driver for Sensirion SHT3x temperature and humidity sensors
connected via I2C.

This commit is a rebasing and contains some interface changes based on
the review from @ourairquality.

examples/sht3x/sht3x_two_sensors/Makefile

@@ -0,0 +1,3 @@
+PROGRAM=SHT3x_Two_Sensors
+EXTRA_COMPONENTS = extras/i2c extras/sht3x
+include ../../../common.mk

examples/sht3x/sht3x_two_sensors/sht3x_two_sensors.c

@@ -0,0 +1,135 @@
+/**
+ * Simple example with two SHT3x sensors.
+ *
+ * It shows two different user task implementations, one in *single shot mode*
+ * and one in *periodic mode*. In addition, it shows both approaches for the
+ * implementation of waiting for measurement results, one as busy waiting and
+ * one as passive waiting using *vTaskDelay*.
+ * 
+ * Harware configuration:
+ *
+ *   +------------------------+     +----------+
+ *   | ESP8266  Bus 0         |     | SHT3x_1  |
+ *   |          GPIO 5 (SCL)  ------> SCL      |
+ *   |          GPIO 4 (SDA)  ------- SDA      |
+ *   |                        |     +----------+
+ *   |          Bus 1         |     | SHT3x_2  |
+ *   |          GPIO 14 (SCL) ------> SCL      |
+ *   |          GPIO 12 (SDA) ------- SDA      |
+ *   +------------------------+     +----------+
+ */
+ 
+#include "espressif/esp_common.h"
+#include "esp/uart.h"
+
+#include "FreeRTOS.h"
+#include "task.h"
+
+// include SHT3x driver
+
+#include "sht3x/sht3x.h"
+
+// define I2C interfaces at which SHTx3 sensors are connected
+
+#define I2C_1_BUS       0
+#define I2C_1_SCL_PIN   GPIO_ID_PIN((5))
+#define I2C_1_SDA_PIN   GPIO_ID_PIN((4))
+
+#define I2C_2_BUS       1
+#define I2C_2_SCL_PIN   GPIO_ID_PIN((14))
+#define I2C_2_SDA_PIN   GPIO_ID_PIN((12))
+
+static sht3x_sensor_t* sensor1;
+static sht3x_sensor_t* sensor2;
+
+/*
+ * User task that triggers measurements of sensor1 every 5 seconds. Due to
+ * power efficiency reasons, it uses the SHT3x *single_shot* and *vTaskDelay*
+ * to wait for measurement results.
+ */
+void user_task_sensor1 (void *pvParameters)
+{
+    sht3x_values_t values;
+    int32_t duration;
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+
+    while (1) 
+    {
+        // trigger one measurement in single shot mode
+        duration = sht3x_start_measurement (sensor1, single_shot);
+
+        // passive waiting until measurement results are available
+        if (duration > 0)
+            vTaskDelay (duration/portTICK_PERIOD_MS);
+        
+        // retrieve the values and do something with them
+        if (sht3x_get_results (sensor1, &values))
+            printf("%.3f SHT3x Sensor1: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3,  
+                   values.temperature, values.humidity);
+
+        // passive waiting until 5 seconds are over
+        vTaskDelayUntil(&last_wakeup, 5000 / portTICK_PERIOD_MS);
+    }
+}
+
+/*
+ * User task that fetches latest measurement results of sensor2 every 2
+ * seconds. It starts the SHT3x in periodic mode with 10 measurements per
+ * second (*periodic_10mps*). It uses busy waiting for first measurement
+ * results.
+ */
+void user_task_sensor2 (void *pvParameters)
+{
+    sht3x_values_t values;
+
+    // start periodic measurement mode
+    sht3x_start_measurement (sensor2, periodic_10mps);
+
+    // busy waiting until measurement results are available
+    while (sht3x_is_measuring (sensor2) > 0);
+
+    TickType_t last_wakeup = xTaskGetTickCount();
+    
+    while (1) 
+    {
+        // retrieve the values and do something with them
+        if (sht3x_get_results (sensor2, &values))
+            printf("%.3f SHT3x Sensor2: %.2f °C, %.2f %%\n", 
+                   (double)sdk_system_get_time()*1e-3,  
+                   values.temperature, values.humidity);
+
+        // passive waiting until 2 seconds are over
+        vTaskDelayUntil(&last_wakeup, 2000 / portTICK_PERIOD_MS);
+    }
+}
+
+
+void user_init(void)
+{
+    // Please note: Return values are not considered in this example for 
+    // readability reasons. All functions return boolean and set an error
+    // code that allows effective error handling.
+    
+    // Set UART Parameter
+    uart_set_baud(0, 115200);
+
+    // Give the UART some time to settle
+    sdk_os_delay_us(500);
+    
+    // Init I2C bus interfaces at which SHT3x sensors are connected
+    // (different busses are possible)
+    i2c_init(I2C_1_BUS, I2C_1_SCL_PIN, I2C_1_SDA_PIN, I2C_FREQ_100K);
+    i2c_init(I2C_2_BUS, I2C_2_SCL_PIN, I2C_2_SDA_PIN, I2C_FREQ_100K);
+
+    // Create sensors
+    sensor1 = sht3x_init_sensor (I2C_1_BUS, SHT3x_ADDR_2);
+    sensor2 = sht3x_init_sensor (I2C_2_BUS, SHT3x_ADDR_1);
+    
+    // Create tasks that use the sensors
+    xTaskCreate(user_task_sensor1, "user_task_sensor1", 256, NULL, 2, 0);
+    xTaskCreate(user_task_sensor2, "user_task_sensor2", 256, NULL, 2, 0);
+        
+    // That's it.
+}