fftshift! and ifftshift!

[Gregstrq]

Adds nonallocating versions of fftshift and ifftshift with the corresponding tests. Fixes #62

[codecov]

Codecov Report

Merging #66 (bdcf670) into master (4f630d6) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   82.75%   82.92%   +0.16%     
==========================================
  Files           2        2              
  Lines         203      205       +2     
==========================================
+ Hits          168      170       +2     
  Misses         35       35              

Impacted Files	Coverage Δ
src/definitions.jl	65.34% <100.00%> (+0.70%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4f630d6...bdcf670. Read the comment docs.

[giordano]

Looks overall ok to me, but probably for consistency the allocating versions should be based on the non-allocating ones? Also, add backreferences in the docstrings of the allocating functions. However I'm not familiar with the internals of this package. Anyone else who can give another review pass? Maybe @stevengj?

[Gregstrq]

@giordano Sorry for the delay. I've implemented your suggestions.

[Gregstrq]

I have added all your suggestions, @stevengj.