Add voltage reading of the battery

[ClemensGruber]

The battery is over a voltage divider connected to one pin of the FiPY. I think we have different voltage dividers for the

• Expansion board
• PCB

We should measre the voltage and calculate the "real" voltage of the LiPo and submit this data in the dataset also to notify user in case the battery is going down.

[amotl]

Hi there,

we just completed reasonable implementations of the most important system sensors we have been aiming at and reported about it at hiveeyes/terkin-datalogger#5.

Cheers,
Andreas.