sdc function does not work for directional nfft

```julia
using NFFT, NFFTTools

J = 11

y = rand(J) .- 0.5
N = (16,20)
p1 = plan_nfft(y, N, dims=1)
f = randn(ComplexF64, N)
fHat = p1 * f

w=sdc(p1)
```

error :
```
MethodError: no method matching sdc(::NFFT.NFFTPlan{Float64, 2, 2})
Closest candidates are:
sdc(!Matched::AbstractNFFTs.AbstractNFFTPlan{T, D, 1}; iters) where {T, D}
@ NFFTTools ~/.julia/packages/NFFTTools/UlGIF/src/samplingDensity.jl:2
```