-------------------------------------------------------------------------------
   Design an electric propulsion system.
   Uses the payload mass, the required delta-V, the required
   thrust, the ratio of structure to fuel mass (for tanks, etc.) and
   the power per unit mass.

   Type ElectricPropulsionDesign for a demo.
-------------------------------------------------------------------------------
   Form:
   x = ElectricPropulsionDesign( x )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   x		  (:)		Data structure
                       x.dV     Total delta V capability (m/sec)
                       x.uE     Exhaust velocity (m/sec)
                       x.pR     Power to propulsion system mass ratio (w/kg)
                       x.sR     Structural mass to fuel ratio
                       x.mP     Payload mass (kg)
                       x.thrust Thrust (N)

   -------
   Outputs
   -------
   x             (:)   Data structure additions
                       x.mT     Total mass
                       x.mS     Structural mass
                       x.mR     Propulsion system mass
                       x.mF     Fuel mass
                       x.power  Total power (w)

-------------------------------------------------------------------------------