fix freqkernel

[JeffFessler]

Addresses #206
The substance here is the changed lines with wrapindex and the corresponding added test.
There is also a subtle point that the kernel needs to be supported in [-N/2, N/2-1) so I added checks for that too (handling both even and odd N cases). And some minor tweaking of docstrings.

FWIW, I'm teaching a graduate image processing course where I am trying to translate everything from Matlab to Julia so getting this psf2tof equivalent working is very desirable.

[codecov]

Codecov Report

Merging #207 (e57a5b3) into master (b4f3b7b) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   91.07%   91.11%   +0.03%     
==========================================
  Files          10       10              
  Lines        1412     1418       +6     
==========================================
+ Hits         1286     1292       +6     
  Misses        126      126              

Impacted Files	Coverage Δ
src/ImageFiltering.jl	77.27% <ø> (ø)
src/mapwindow.jl	86.28% <100.00%> (+0.06%)	⬆️
src/utils.jl	88.23% <100.00%> (+0.93%)	⬆️

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[JeffFessler]

Hi @johnnychen94 are you a maintainer here? This PR has been ready for a while.
Anything else I should do?

[timholy]

Sorry for the delay. It looks quite good, just a stylistic point (optional) and suggestion for testing your error conditions too.

[JeffFessler]

I just learned about Aqua.jl and tested it out on ImageFiltering.jl and it reported that ImageMetadata is unused so I removed it from deps. This is a bit off topic from freqkernel but figured I'd include it in this PR anyway and see how it goes...

[JeffFessler]

Also addressing #208 with a 1-line fix. Hopefully.

[timholy]

Very nice work, @JeffFessler. Thanks!

[timholy]

JuliaRegistries/General#30822

[johnnychen94]

I was AFK for the last two months so sorry for the inactivity. I've looked at the psf2otf source codes two years ago so I'm contaminated and thus can't review this PR (#81). Thank you both for the efforts.