Add option to suppress verbose midi conversion output

[ohag487]

Summary

• Converting a midi file to wav via Timidity (midi_to_wav in chord_extractor/converters.py) previously always let Timidity's console output through unfiltered. Timidity writes fairly verbose diagnostic info about the midi file's internals (e.g. track/lyric/text events) to stderr, which was not being captured or suppressible, so it prints straight to the console during conversion.
• Adds an opt-in suppress_output parameter to midi_to_wav, which redirects Timidity's stderr to DEVNULL when set.
• Threads this through as suppress_midi_output on ChordExtractor.__init__ (used in preprocess) and Chordino.__init__, so callers can do Chordino(suppress_midi_output=True) to silence the conversion noise.
• Default behaviour is unchanged (suppress_midi_output=False) to avoid any breaking change for existing consumers.
• Updated docstrings and README with a short usage note.

Test plan

• Added tests/test_converters.py covering that suppress_output=True passes stderr=DEVNULL to the Timidity subprocess call, and that the default leaves stderr untouched (mocks subprocess.run, so no Timidity binary needed).
• python -m pytest tests/test_converters.py -v
• flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics (no new issues; also fixed a pre-existing long-line warning on the touched line)
• Existing tests/test_chordino.py::test_extract_many (requires Timidity binary + Chordino native lib, not available in this sandbox — behavior unaffected since new option defaults to off)

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