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().
Also change the `tester` package slightly to use an exposed `Registers`
array rather adding yet another accessor method to retrieve a register
value. This seems to me more transparent and "obvious" - we aren't trying
to hide the fact that there's just a simple memory store there.
Also unexport the `assertRegisterRange` method which was never intended
to be part of the public API.