don't return chunk if it's too small
don't return chunk if it's too small

jaduimstra/nilmtk,pauldeng/nilmtk,AlexRobson/nilmtk,HarllanAndrye/nilmtk,josemao/nilmtk,mmottahedi/nilmtk,nilmtk/nilmtk,nilmtk/nilmtk