read wave specifier

ReadWaveSpecifier.cc

void ReadWaveSpecifier(){
    std::string wav_rspecifier = "scp:data/wav.scp";
    SequentialTableReader<WaveHolder> wav_reader(wav_rspecifier);
    for (; !wav_reader.Done(); wav_reader.Next()){
        std::string key = wav_reader.Key();
        WaveData wave_data = wav_reader.Value();
        cout << "wav_spec dur: "<<wave_data.Duration()<<endl;
    }
}