Make it possible to use a cu:: object as a member variable

[csbnw]

Is your feature request related to a problem? Please describe.
It is currently not possible to have e.g. a cu::Stream as a member variable in another class without using std::unique_ptr.

Describe the solution you'd like
Fix this.

Describe alternatives you've considered
N.A.

Additional context
See also here.

[matmanc]

This relates to issue #167