Add function that returns temperature as Celsius as a float

[Doridian]

Returning temperature as C*4 might make sense data-wise and for internal processing, as that is the precision the chip works in, however it doesn't make sense for use as a display. And since the LSB is the decimal part and the MSB is the integral part, I think it makes sense adding that directly as a library function. Maybe also add a "temperatureFahrenheit" lateron?

[JChristensen]

That code will return incorrect values for temperatures < 0°C.