Fix tortuosity CSV export

During the last refactor, the array overzealously removed. It is needed by
the export csv code.
The array was re-installed and documentation updated to prevent this
problem from happening again. Also made the code more robust and more
explicit by checking for possibles failures more often.
Merge branch 'FixTortuosityExportCSV'

* FixTortuosityExportCSV:
  Fix tortuosity CSV export