[WIP] refactor filtering and scaling of spectrograms

[superbock]

Changes proposed in this pull request

This PR refactors the filtering and scaling of spectrograms to be more memory efficient. Memory savings are mostly due to casting several objects to numpy arrays earlier than before and hence freeing memory by discarding references.

Additional options for stft() allow further memory savings, e.g. by computing only real-valued magnitude spectrograms (via complex=False) or filtering the magnitude spectrogram by passing a filterbank and perform this filtering in blocks (by passing block_size).

The changes come with an API change of FilterbankProcessor (which was never used anywhere in our own code).

This pull request fixes #248, and mitigates #404.

Remaining TODOs before this pull request can be merged

• add literal options for filterbank (e.g. Mel, Bark, logarithmic, semitone)
• add more scaling options (e.g. dB, perceptual weighting)
• rethink default values of ScalingProcessor
• decide if complex and circular_shift could/should be unified
• block_size is used already by StrideLayer, but this layer does not accept additional kwargs, thus there is no direct naming conflict, still it is not optimal

[codecov]

Codecov Report

Merging #405 into master will decrease coverage by 0.02%.
The diff coverage is 96.01%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
- Coverage   94.61%   94.59%   -0.03%     
==========================================
  Files          93       93              
  Lines       15107    15202      +95     
==========================================
+ Hits        14294    14380      +86     
- Misses        813      822       +9

Impacted Files	Coverage Δ
madmom/features/key.py	100% <100%> (ø)	⬆️
madmom/features/beats.py	90.78% <100%> (+0.03%)	⬆️
bin/GMMPatternTracker	96.36% <100%> (+0.06%)	⬆️
madmom/features/notes.py	96.55% <100%> (+0.25%)	⬆️
tests/test_audio_filters.py	100% <100%> (ø)	⬆️
madmom/features/downbeats.py	95.52% <100%> (+0.1%)	⬆️
madmom/features/chords.py	100% <100%> (ø)	⬆️
bin/LogFiltSpecFlux	94.11% <100%> (+0.17%)	⬆️
bin/SuperFluxNN	95.23% <100%> (+0.11%)	⬆️
madmom/audio/spectrogram.py	92.36% <100%> (+0.27%)	⬆️
... and 18 more

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