Two small download_HHG fixes.

Hey,

as discussed with Eugen I forked the repo and added my quick-fixes for issues I had with the download_HHG script. Since I have no experience with GitHub at all, let me know if you think there's a better way to feed back changes to the project.

The first issue was that the download_HHG script did not work if there was only one logfile on the HedgeHog, the second that in some cases the last logfile did not have any 'fta' (?), which seems to be important for splitting the data into daily numpy files.

If there's any questions, just let me know at TU. I just did my best with yet-another-new-tool.

Best, Jan