all: introduce a temperature type

This type should be used whenever a sensor (or actuator?) works with a
temperature. For example, this commit changes the signature:

    ReadTemperature() (int32, error)

to the following:

    ReadTemperature() (drivers.Temperature, error)

I believe this is much clearer in intent. It also makes it trivial to
introduce common conversions. For example, there are already Celsius()
and Fahrenheit() methods to convert to the given units, as a floating
point. More units could be added as needed, for example a CelsiusInt().

sht3x/sht3x.go

@@ -31,9 +31,9 @@ func New(bus drivers.I2C) Device {
 }
 
 // Read returns the temperature in celsius milli degrees (°C/1000).
-func (d *Device) ReadTemperature() (tempMilliCelsius int32, err error) {
+func (d *Device) ReadTemperature() (tempMilliCelsius drivers.Temperature, err error) {
 	tempMilliCelsius, _, err = d.ReadTemperatureHumidity()
-	return tempMilliCelsius, err
+	return drivers.Temperature(tempMilliCelsius), err
 }
 
 // Read returns the relative humidity in hundredths of a percent.
@@ -43,15 +43,15 @@ func (d *Device) ReadHumidity() (relativeHumidity int16, err error) {
 }
 
 // Read returns both the temperature and relative humidity.
-func (d *Device) ReadTemperatureHumidity() (tempMilliCelsius int32, relativeHumidity int16, err error) {
+func (d *Device) ReadTemperatureHumidity() (tempMilliCelsius drivers.Temperature, relativeHumidity int16, err error) {
 	var rawTemp, rawHum, errx = d.rawReadings()
 	if errx != nil {
 		err = errx
 		return
 	}
-	tempMilliCelsius = (35000 * int32(rawTemp) / 13107) - 45000
+	tempMilliCelsius = drivers.Temperature((35000 * int32(rawTemp) / 13107) - 45000)
 	relativeHumidity = int16(2000 * int32(rawHum) / 13107)
-	return tempMilliCelsius, relativeHumidity, err
+	return
 }
 
 // rawReadings returns the sensor's raw values of the temperature and humidity