add millisecond precision to timekeeping

[KravitzLab]

run a timer that resets every second using the RTC, and use millis() to add millisecond precision to the timekeeping functions

[KravitzLab]

Add simple millis() column to log file

[KravitzLab]

This is also possible by using LowPower.idle() instead of LowPower.sleep(). Need to do power consumption measurements in both cases.

https://circuitdigest.com/microcontroller-projects/arduino-sleep-modes-and-how-to-use-them-to-reduce-power-consumption

https://github.com/arduino-libraries/ArduinoLowPower/blob/master/src/ArduinoLowPower.h

[KravitzLab]

Also I think LowPower.deepsleep() is just LowPower.sleep(), will confirm with Otii measurement