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   Computes pressure ratio from temperature ratio, efficiency and gamma.
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   Form:
   pi = PressureRatioFromTempRatio( tau, eta, gamma )
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   Inputs
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   tau        (1,n)  Temperature ratio
   eta        (1,n)  Efficiency
   gamma      (1,n)  Gamma

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   Outputs
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   pi         (1,n)   Pressure ratio

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