Set API for Unwrapping of 1D/0D Scalars

[ejmeitz]

All reductions return 0D or 1D NDArrays currently instead of Julia scalars. This means that things like

arr = cuNumeric.ones(5)
if all(arr)
   ...
end

do not work because all(arr) is not a Bool.

We have functions like unwrap and Base.only which materialize the result.

It might also be possible to define convert functions so that if a user tries to pass a 1D/0D NDArray to a function expecting a scalar we can auto-convert it.