flatten/flatten_view/ref
Merge pull request #4 from jamboree/develop

flatten/flatten_view/ref
fixes #5208
put struct nil_ in its own header to fix incomplete type error with nvcc
fix copyright
Merge pull request #5 from ericniebler/nil-hpp

Move definition of struct nil_ into its own header
reverting support for ref pending review
Merge branch 'origin/develop'
Merge branch 'develop'