Fix COMBED dataset converter for pandas>=0.23 (due to the bugfix pandas-dev/pandas#19497). Still compatible with 0.22 by using `pd.MultiIndex` directly.
Fix COMBED dataset converter for pandas>=0.23 (due to the bugfix pandas-dev/pandas#19497). Still compatible with 0.22 by using `pd.MultiIndex` directly.

nilmtk/nilmtk,nilmtk/nilmtk