SdFat flush instead of delay

[awickert]

No more guesswork

  if (first_log_after_booting_up){
    end_logging_to_headerfile();
    first_log_after_booting_up = false; // the job is done.
  }
  delay(30);
  // THIS DELAY IS ***CRITICAL*** -- WITHOUT IT, THERE IS NOT SUFFICIENT
  // TIME TO WRITE THE DATA TO THE SD CARD!
  // Check right before going back to sleep if there has been a rain
  // gauge bucket tip while it has been on
  // This is a temporary solution!
  // (May be able to reduce delay if not going back to sleep -- i.e., write
  //  to card while logging next step.)