Fixing MeterGroup.mains() and ElecMeter.upstream_meter().

* elecmeter.upstream_meter() returns ElecMeter or MeterGroup
* elecmeter.__init__ adds itself to instance=0 meter group if is_site_meter()
* metergroup.mains() returns a MeterGroup of all meters
  where `meter.is_site_meter()` == True
* raise NotImplementedError if 'upstream_meter_in_building' is used.