fixing SettingWithCopyWarning

In [45]: ds.buildings[1].elec.proportion_of_energy_submetered()
number of good sections = 26
/home/dk3810/workspace/python/nilmtk/nilmtk/pipeline/clipnode.py:25: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead