remote bus reactive power control for generators

[abhyshr]

Implement remote bus reactive power control. This will need addition of new bus types:

• Distributed P bus: These buses are generator buses that are controlling the reactive power for a common remote bus. Note that in the data file, these buses are specified as PV buses. Since the generators at these buses are controlling the voltage at a remote bus, they are not actually PQ buses, i.e., the voltage at these buses is not controlled.
• PQV bus: The remote bus (whose reactive power controlled by generators at other buses) will have a type PQV where P, Q, and V are given. The only variable being solved by power flow at this bus is the voltage angle $\theta$
  @yliu250