Suggestion: infer plan type from `x` type

If I understand correctly, the current approach is that user decides between cpu and gpu versions by either
* CPU: `p = plan_nfft(x, N)` or (currently equivalently I think) `p = plan_nfft(Array, x, N)`
* GPU: `p = plan_nfft(CuArray, x, N)`

I'd prefer that the default plan type come from the type of `x` by adding a method something like
`plan_nfft(x, N) = plan_nfft(typeof(x), x, N)`

So if `x` is a `CuArray` then by default the plan will be a GPU plan.

The reason is that then I can embed `plan_nfft` into downstream packages without forcing them to depend on CUDA.jl.
This might also "future proof" it so that someday when we have a OpenCLArray version then again it can inherit from `x`.
But I might be missing something?
And I have not really been able to test it out yet because I think the recent CUDA fixes are waiting for release updates.

I realize that if `x` is an `Adjoint` type (or a Range) then this would need additional work to get at the "base" of such types.