This sensor determines the continuous quasi RMS value of a
multi phase voltage system, by averaging the phase RMS voltage
phasors v.
  V = sum({'abs'(v[k]) for k in 1:m})/m
ReferenceSensor, FrequencySensor, PotentialSensor, VoltageSensor, VoltageQuasiRMSSensor, CurrentSensor, CurrentQuasiRMSSensor, PowerSensor, MultiSensor