EP2M

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   Converts structural and payload ratios into masses.
   1 2 3 4 ... payload
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   Form:
   [m0, mf] = EP2M( e, p, mpayload )
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   Inputs
   ------
   e            (1,:) Stage structural ratios
   p            (1,:) Stage payload ratios
   mpayload     (1,1) Payload mass

   -------
   Outputs
   -------
   m0           (1,:) Total mass of each stage (including everything above)
   mf           (1,:) Final mass of each stage (including everything above)

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    References: Wiesel, W. E., Spaceflight Dynamics, McGraw-Hill, 1988,
                pp. 199-202.
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