MSThrustModel:

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   Simple thrust model for multi-stage vehicle.

   Stages fire sequentially. The current mass and flow rate are
   calculated. Coast times may be inserted between the stages. To ignore
   coast times and burn continuously, the last two inputs may be left out.

   This model does not handle parallel solid boosters. To incorporate boosters, 
   see LVCombineSRB.

   Since version 2015.1
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   Form:
   [thrust,mass,mDot,j] = MSThrustModel( mFuel, rocket, t, coast )
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   Inputs
   ------
   mFuel       (:,1)    Fuel mass states
   rocket       (.)     Rocket model
                         .mSS      Structural (dry) mass of the stages
                         .thrust   Thrust of the stages (vacuum)
                         .uE       Exhaust velocity (vacuum)
                         .tBurn    Burn time of stage
   t           (1,1)    Current elapsed time*
   coast       (1,:)    Coast times (sec)*
                          * Optional, required only for coasting

   -------
   Outputs
   -------
   thrust      (1,1)    Current thrust
   mass        (1,1)    Vehicle current mass
   mDot        (:,1)    Mass derivative
   j           (1,1)    Current stage index

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