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   Computes the dimensionless total temperature.
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   Form:
   theta0 = Theta0( gamma, t0, machNo, units )
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   Inputs
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   gamma           (1,n)   Burner temperature at sea level static
   t0              (1,n)   Temperature
   machNo          (1,m)   Mach number
   units           (1,:)   'mks' or 'eng'

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   Outputs
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   theta0          (n,m)   Dimensionless total temperature

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