`MeterGroup.meters_correlation()` now uses full `ElecMeterIDs` as the
column and row labels for the returned DataFrame. #160.  Also:

* modified `plot_correlation_heatmap` to accept a `labels` argument
* modified notebook to use labels for correlation heatmap
* modified `ElecMeter.appliance_label` so it outputs 'SITE METER' for
  site meters.