more efficient and robust track logging

[Martin Kolman]

• current setup
  • currently the whole track is periodically dumped to a GPX file
  • that means that the longer logging is running, the longer takes it takes to save the tracklog

• efficiency
  • just save the increment to a simple text file with one line per point (CSV)
  • periodically save small increments (say 10 lines every 10 seconds for 1 point per second logging frequency)
  • once logging is stopped, convert the simple text files to GPX
  • check on startup if there are some files left from the last time (in case modRana couldn't exit cleanly)

• robustness
  • save to two files so that if one gets corrupted (eq. modRana or device crashing while the file is being written), there is still one left