Add normalizing flows - first draft

[alicjapolanska]

Draft implementation of normalizing flow based models. NormalizingFlow class will be deleted and RQSpline will be refactored to follow the same structure as RealNVP.

[codecov-commenter]

Codecov Report

Patch coverage: 1.53% and project coverage change: -18.72 ⚠️

Comparison is base (0244139) 93.38% compared to head (2dea7a8) 74.66%.

❗ Current head 2dea7a8 differs from pull request most recent head 6f12d46. Consider uploading reports for the commit 6f12d46 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #228       +/-   ##
===========================================
- Coverage   93.38%   74.66%   -18.72%     
===========================================
  Files           6        8        +2     
  Lines        1028     1287      +259     
===========================================
+ Hits          960      961        +1     
- Misses         68      326      +258     

Impacted Files	Coverage Δ
harmonic/flows.py	0.00% <0.00%> (ø)
harmonic/model_nf.py	0.00% <0.00%> (ø)
harmonic/evidence.pyx	94.52% <66.66%> (-0.83%)	⬇️

... and 1 file with indirect coverage changes

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