fix: correct whitening in HilbertCPCCA models

[nicrie]

During the data preprocessing of the Hilbert CPCCA model the data was first whitened and then Hilbert transformed. The final Hilbert transform destroyed the whitening property of the data. The correct order should be Hilbert transform -> Whitening.

In order to fix this we refactor the Whitener into two classes, a PCA preprocessor and a pure Whitener. Then we move the data augmentation step in between the PCA and Whitener step.

[codecov]

Codecov Report

Attention: Patch coverage is 16.44737% with 127 lines in your changes missing coverage. Please review.

Please upload report for BASE (develop@bcbaa9e). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
xeofs/preprocessing/pca.py	27.39%	53 Missing ⚠️
xeofs/cross/cpcca.py	0.00%	26 Missing ⚠️
xeofs/cross/base_model_cross_set.py	5.00%	19 Missing ⚠️
xeofs/preprocessing/whitener.py	8.33%	11 Missing ⚠️
xeofs/cross/cpcca_rotator.py	9.09%	10 Missing ⚠️
xeofs/single/pop.py	11.11%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #230   +/-   ##
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  Coverage           ?   24.58%           
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  Files              ?       59           
  Lines              ?     4047           
  Branches           ?        0           
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  Hits               ?      995           
  Misses             ?     3052           
  Partials           ?        0           

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