#1 Sensors should send their battery voltage alongside measurement results

Sensors should send their battery voltage alongside measurement results. Required changes: - [ ] A new measurement type needs to be added to the protocol (battery state in percent?). - [ ] The sensors need to be modified to measure the battery state. The ATTiny ADC can be configured to measure the bandgap voltage (1.1V) relative to VCC, so the inverse is proportional to the battery voltage.