Fix bug in radialspectrum()

[parfenyev]

Fixing bug in the function radialspectrum(). Now the definitions of lshift and kshift are consistent with the operation fftshift.

[navidcy]

I suggest using the grid's wavenumber and shifting them using FFTW.fftshift instead of re-creating them. This way we minimize the places where errors can be introduced.

Also, I'd suggest changing

kshift = range(0, stop=grid.nkr-1, length=grid.nkr) * 2π/grid.Lx

to

kshift = grid.kr

[navidcy]

You'd probably need to import fftshift, e.g., add it in

https://github.com/parfenyev/FourierFlows.jl/blob/ebeb30c9fed132a97e3bf19870ccd3f18290dc15/src/FourierFlows.jl#L77

[parfenyev]

In your version I have the problem with interpolation:

MethodError: no method matching scale(::Interpolations.BSplineInterpolation{ComplexF64, 2, Matrix{ComplexF64}, BSpline{Linear{Throw{OnGrid}}}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}, ::Matrix{Float64}, ::StepRangeLen{Float64, Base.TwicePrecision{Float64}, Base.TwicePrecision{Float64}, Int64})
Closest candidates are:
  scale(::AbstractInterpolation{T, N, IT}, ::AbstractRange...) where {T, N, IT} at ~/.julia/packages/Interpolations/y4lLj/src/scaling/scaling.jl:27

Probably, it happens because interpolation works only on uniformly spaced grids and this is the way to control this.

[navidcy]

Yeap, fair. Forget my suggestion then! :)

[navidcy]

Thanks @parfenyev!