No tests for nufft routines

[gwater]

Tests only cover nudft routines which are not exported/acvertised in documentation. As a result nufft2 is unusable without any indication from tests.

[MikaelSlevinsky]

Not sure that's true. This test uses the direct transforms to establish a comparison https://github.com/JuliaApproximation/FastTransforms.jl/blob/8625a440488c79e40355ab966bb805dc9650cfde/test/nuffttests.jl#L50-L52

[gwater]

It looks like nufft2 works now and there is some minimal documentation.

To prevent nufft2 from breaking again in the future, I nevertheless recommend some direct test coverage.