Load a set of data from a structured folder in the following format:
        {prefix}/
            wake-word/
                *.wav
            not-wake-word/
                *.wav
            test/
                wake-word/
                    *.wav
                not-wake-word/
                    *.wav

def from_folder(cls, folder: str) -> 'TrainData':
        """
        Load a set of data from a structured folder in the following format:
        {prefix}/
            wake-word/
                *.wav
            not-wake-word/
                *.wav
            test/
                wake-word/
                    *.wav
                not-wake-word/
                    *.wav
        """
        return cls(find_wavs(folder), find_wavs(join(folder, 'test')))