updated notation to use x for appliance states instead of z
updated notation to use x for appliance states instead of z

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