@ladyada says:
"having this be adjustable (reference) would be ideal cause you can get
absolute voltages but for now, VCC/4 + 4x matches every other chip :)"
... and indeed doing it this way happens to give a much more steady
reading when using a VCC-referenced resistance, and so many of the simple
things you'd wire up are actually VCC-referenced anyway.
The nRF MCUs do not support analog output. Throwing an exception
in the constructor will stop users from creating an instance of
the AnalogOut class.
In the future we can ifdef-out the whole class so it is not available
in the module at all.
