FFT computation

FFTManager.m

    NSError *computationError;
    AVAudioFile *audioFile = [[AVAudioFile alloc] initForReading:[NSURL fileURLWithPath:self.filePath] error:&computationError];
    if (computationError) {
        failure(computationError);
        return;
    }
    
    AVAudioFormat *audioFormat = audioFile.processingFormat;
    UInt32 audioFrameCount = (UInt32) audioFile.length;
    AVAudioPCMBuffer *buffer = [[AVAudioPCMBuffer alloc] initWithPCMFormat:audioFormat frameCapacity:audioFrameCount];