Add region(::Plan) for accessing transformed region

[devmotion]

Currently, there is no official and consistent way for accessing the transformed region of a FFT plan. For many plans, plan.region works but e.g. it fails for ScaledPlan. This is problematic when one tries to write generic implementations, e.g., of derivatives (see, e.g., JuliaMath/FFTW.jl#182).

This PR adds a function region(p::Plan) that can be used to access the region of a plan. It falls back to p.region but can be specialized, e.g., for ScaledPlan.

[codecov]

Codecov Report

Merging #65 (8c4dcd9) into master (6434327) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   82.92%   83.09%   +0.16%     
==========================================
  Files           2        2              
  Lines         205      207       +2     
==========================================
+ Hits          170      172       +2     
  Misses         35       35              

Impacted Files	Coverage Δ
src/definitions.jl	66.01% <100.00%> (+0.67%)	⬆️

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[gaurav-arya]

This should be merged before #67 or into #67?

[devmotion]

This should be merged before #67 or into #67?

IMO it would be cleaner to keep this change separate.

[gaurav-arya]

Sounds good -- since I use region in my PR it would be good to merge this first then. @stevengj would you be able to take a look?

[stevengj]

Thanks!

I renamed region(p) to fftdims(p) — the name dims seems more common nowadays in Julia for this kind of thing (see e.g. mapslices, sum, etcetera), and since this is an exported API we have to be more careful about the naming.

(We may want to go back and change the name region to dims in the docs at some point. No need to change the region field of plans, however, as that is a private field.)