copying of CUDA Plan does not work

Sorry for spamming :laughing:

But copy of a CUDA plan does not work:
```
p = NFFT.plan_nfft(coords, (size(x,1), size(x,2)))

MethodError: no method matching copy(::CuNFFT.CuNFFTPlan{Float32, 2})

Closest candidates are:

copy(!Matched::LinearAlgebra.Hessenberg{<:Any, <:LinearAlgebra.UpperHessenberg}) at ~/.julia/juliaup/julia-1.8.5+0.x64.linux.gnu/share/julia/stdlib/v1.8/LinearAlgebra/src/hessenberg.jl:419

copy(!Matched::LinearAlgebra.Hessenberg{<:Any, <:LinearAlgebra.SymTridiagonal}) at ~/.julia/juliaup/julia-1.8.5+0.x64.linux.gnu/share/julia/stdlib/v1.8/LinearAlgebra/src/hessenberg.jl:420

copy(!Matched::LinearAlgebra.Cholesky) at ~/.julia/juliaup/julia-1.8.5+0.x64.linux.gnu/share/julia/stdlib/v1.8/LinearAlgebra/src/cholesky.jl:511

...

(::Main.var"workspace#17".var"#1#2"{CuNFFT.CuNFFTPlan{Float32, 2}})(::Int64)@none:0
iterate@generator.jl:47[inlined]
collect@array.jl:787[inlined]
f_cuda@[Other: 3](http://localhost:1234/edit?id=96cda76c-d92d-11ed-0c54-bb1f68c531e4#)[inlined]
top-level scope@[Local: 1](http://localhost:1234/edit?id=96cda76c-d92d-11ed-0c54-bb1f68c531e4#)[inlined]
```