Automatically write linkstats to output folder

[markusstraub]

It would improve my workflow if the linkstats were already contained in the output folder along all other relevant outputs such as output_events.xml.gz. As it is now the linkstats are only located in the respective iteration folders.

The small proposed change would also write an output_linkstats.txt.gz to the output folder if (and only if) the last iteration produced a linkstats.txt.gz in the repsective iteration folder.

Small sidenote: It guess it would make sense to also change the linkstats output to .csv as all other outputs.