ENH: Add streamline concatenation script

[jhlegarreta]

Add streamline concatenation script and the necessary methods into the appropriate module.

[jhlegarreta]

These 2 scripts may be doing exactly what is proposed here:
https://github.com/SlicerDMRI/whitematteranalysis/blob/master/bin/wm_append_clusters_to_anatomical_tracts.py
https://github.com/SlicerDMRI/whitematteranalysis/blob/master/bin/wm_append_clusters.py

Maybe they can be refactored so that the "append" operation can be reused across modules.

[jhlegarreta]

OK, so wm_append_clusters.py does this job (thanks Fan); the point-wise data is also preserved; the only scalar that is not preserved is the cluster idx, which is generated again and considers each tractogram as one cluster (whereas each input tractogram may contain different clusters). Refactoring the concatenation/append operation into a method is left for future work. So closing the PR for now.