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   Computes the REA and tank pressure
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   Form:
   [p,pSystem] = Pressure(mFuel,database,temp)
   [p,pSystem] = Pressure(mFuel,database)
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   ------
   Inputs
   ------
   mFuel          (1,n)   Propellant mass per system
   database       (1,:)   Spacecraft properties database
   temp           (1,n)   System temperature

   -------
   Outputs
   -------
   p              (1,m)   Thruster pressure
   pSystem        (1,n)   System pressure

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