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   Computes the Mach number from pressure ratio.
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   Form:
   m = MachFromPressureRatio( gamma, pR, tol )
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   Inputs
   ------
   gamma          (1,1)       Ratio of specific heats
   pR             (1,:)       Pressure ratio
   tol            (1,1)       tolerance

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   Outputs
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   m              (1,:)       Mach number
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