Set optional fields

[kristianschneider]

The NMEA standard apperently allows for optional fields.
Without those the data cant parse properly and we run into issues like #10

The challenge is that

 for (int i = 1; i <= sizeof(fps) / sizeof(fps[0]); i++) {
    ok &= fps[i - 1](field_strings + field_offsets[i]);
  }

  if (!ok) {
    return false;
  }

in Line 117 discards the entire message if any field is not valid and then you don't get useful data