Fix silly bug that I introduced into Electric.correlation where both y_s and x_s were calculated for `self`.  Oops!
Fix silly bug that I introduced into Electric.correlation where both
y_s and x_s were calculated for `self`.  Oops!

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