The following license files are associated with this item:

Copyright 2018 by the authors.
Submitted for possible
open access publication
under the terms and conditions of the Creative Commons Attribution CC-BY-NC 4.0 license
Except where otherwise noted, this item's license is described as Copyright 2018 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution CC-BY-NC 4.0 license