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   Updates the fuel mass.
   This function can handle any number of thrusters and any number
   of fuel tanks (i.e. elements of fuelMass.)
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   Form:
   fuelMass = FuelUpdt( fuelUsed, tId, fuelMass, database )
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   ------
   Inputs
   ------
   fuelUsed     (1,n)     Fuel used by thrusters
   tId          (1,n)     Thrusters used
   fuelMass     (1,m)     Fuel mass
   database     (1,:)     Spacecraft properties database

   -------
   Outputs
   -------
   fuelMass     (1,m)     Fuel mass

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