Exchangable NFFT backends

[nHackel]

This PR uses the exchangable AbstractNFFTs PR to allow the NFFTOperator to be defined with different NFFT providers such as NonuniformFFTs:

julia> using NonuniformFFTs, LinearOperatorCollection

# setup parameters

julia> nop = NFFTOp(ComplexF64; shape=(N,N), nodes, symmetrize=false, S = typeof(xop))

julia> typeof(nop.plan)
NonuniformFFTs.NFFTPlan{Float64, 2, PlanNUFFT{ComplexF64, 2, 1, 3, CPU, ...})

[nHackel]

This also introduces a breaking change since it drops the TENSOR precompute flag