Prepared sketch for RTC and data logging

The pins used needed to be moved in order to use the Adafruit datalogging
shield. In addition, a DFC77 receiver (European atom clock) will be used
for precise timestamps.

I removed the panel voltage for the time being because of the common
ground issue. Maybe I'll add it again later.
Added RTC/DCF code

Added code for handling the RTC and the DCF receiver.
Added logging to SD card

The sketch now logs all measurements to a CSV file on the SD card.
Removed duplicate hyphen in date/time field
Added new columns to the CSV, removed Serial

The CSV now also contains the time as unix timestamp and the time in
milliseconds since the last reset of the program.

Removed serial logging from the sketch as this code was only intended for
testing. Also removed disabled code.