Add powersensor3 support
This branch adds an interface to PowerSensor3. It doesn't fully compile yet, at least not for me.
A few issues that would be good to fix:
- make sure the PowerSensor3 (in a new git submodule) gets compiled and that pmt is linked to it
- make sure that HAVE_ARDUINO in pmt.h is actually set (I don't think it is)
- also add HAVE_POWERSENSOR3 in the same way as HAVE_ARDUINO
And of course it still needs to be tested.