Ideal source with constant (active, reactive) power when 'pq' unconnected,
with variable (active, reactive) power when 'pq' connected to a signal-input.
Optional input:
  pq               {p_active, p_reactive}
   pq[1], pq[2]     in SI or pu, depending on choice of 'units'
Frequency: the source has always system-frequency.
Use only if prescibed values for pq are compatible with the properties of the connected system.