Add batched and multi-dimensional rfft/irfft support

[jamesquinlan]

Motivation: add support for AbstractMatrix and higher-dim AbstractArray types in rfft and irfft. It was needed to support workflows in SpeedyTransforms.jl which call plans on 2d views. Previously, generic_rfft only handled AbstractVector giving a MehtodError for matrces.

New Features:

• Batched 1d transfomrs: Compute rffts along any dimension of an array.
• Support transforms over regions (aligns with AbstractFFTs interface.
• Handle plan inversions between real and complex types

Minor Bug Fixes & Improvements:

• plan_rfft stores length of the tranformed dimension instead of the total array length
• Fixed double-scaling bug when normalizing multi-dims array
• Lossened pinv in DummyPlan to allow real-input plans to store complex-output inverses.

Verification:

• added set of tests in test/fft_tests.jl of batached 1d rfft/irfft along different dimensions
• 2d rffts
• plan inversion
• all existing tests passed (with a relaxation of tol = 100eps

[milankl]

Awesome thanks for this effort! Feel free to keep @maximilian-gelbrecht and me updated about this progress. We have recently implemented MatrixSpectralTransform in SpeedyTransforms as it's fast at low resolutions with the side effect of being more type flexible as it only needs a matrix vector multiplication. But being able to use GenericFFT is also great!

[codecov]

Codecov Report

❌ Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.82%. Comparing base (eaae4c0) to head (645f59c).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fft.jl	85.36%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   76.05%   80.82%   +4.77%     
==========================================
  Files           2        2              
  Lines         213      266      +53     
==========================================
+ Hits          162      215      +53     
  Misses         51       51              

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