Lots of changes mostly to do with MeterGroup.groupby and plotting and select_top_k
* MeterGroup now has a 'name' attribute
* implemented MeterGroup.values_for_appliance_metadata_key
* implemented MeterGroup.dominant_appliances
* changed MeterGroup._all_columns_from_kwargs to
  _convert_physical_quantity_and_ac_type_to_cols and made it behave
  like its partner in ElecMeter
* implemented MeterGroup.groupby