Added simple test for `Electric.proportion_of_energy()`
Added simple test for `Electric.proportion_of_energy()`

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