update requirements for ReducerArgumentNonConst

[nmm0]

Parallel reduce requirements say:

If ExecPolicy is TeamThreadRange a “reducing” functor is not allowed and the ReducerArgument must satisfy the Reducer concept or ReducerArgumentNonConst must be a POD type with operator += and operator = or a Kokkos::View

Rather we would like requirements that reflect that the type is:

• default constructible
• copyable
• and "behaves ok when cudaMemcpy is called on it"

[nmm0]

sycl has an applicable requirement "Device copyable": https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec::device.copyable