-------------------------------------------------------------------------------
   Computes the multistage tradeoff ratios.
-------------------------------------------------------------------------------
   Form:
   [stor, ptor, m0, mf] = MSTOR( e, p, v, mpayload )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   e            (1,:) Stage structural ratios
   v            (1,:) Exhaust velocity of each stage
   p            (1,:) Stage payload ratios
   mpayload     (1,1) Payload mass

   -------
   Outputs
   -------
   stor         (1,:) dm*/dmsk-Structural tradeoff ratio
   ptor         (1,:) dm*/dmpk-Payload tradeoff ratio
   m0           (1,1) Initial mass
   mf           (1,1) Final mass

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