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   Computes the ratio of impact to static pressure from Mach number and gamma.
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   Form:
   pR = PressureRatioFromMach( gamma, m )
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   Inputs
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   gamma          (1,1)       Ratio of specific heats
   m              (1,:)       Mach number

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   Outputs
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   pR             (1,:)       Pressure ratio

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