fix bug where it was possible to have to open-ended timeframes in combined()
fix bug where it was possible to have to open-ended timeframes in combined()

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