FIX(nufft): Impossible type restriction for nufft2

[gwater]

It was impossible to run nufft2 because the input has to be real and complex at the same time.

[codecov]

Codecov Report

Merging #131 (cb98aac) into master (8625a44) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   78.41%   78.41%           
=======================================
  Files          13       13           
  Lines        1543     1543           
=======================================
  Hits         1210     1210           
  Misses        333      333           

Impacted Files	Coverage Δ
src/nufft.jl	94.76% <ø> (ø)

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[MikaelSlevinsky]

Looks like a lot of the NUFFT type parameterizations are a bit ad hoc.

Some of the executions would benefit from another type parameter, such as

FastTransforms.jl/src/nufft.jl

Line 84 in 8625a44

function mul!(f::AbstractVector{T}, P::NUFFTPlan{1,T}, c::AbstractVector{T}) where {T}

and others would benefit from removing type parameters entirely

FastTransforms.jl/src/nufft.jl

Line 99 in 8625a44

function mul!(F::Matrix{T}, P::NUFFTPlan{N,T}, C::Matrix{T}) where {N,T}

Would you mind going through the files (nufft.jl and inufft.jl) to check cases in this PR?