ENH: kOmegaSST - added turbulence decay control terms

Based on the reference:
      Spalart, P. R. and Rumsey, C. L. (2007).
      Effective Inflow Conditions for Turbulence Models in Aerodynamic
      Calculations
      AIAA Journal, 45(10), 2544 - 2553.

The decay control default is off for backwards compatibility.  To enable
it, add the following to the coefficients dictionary

      // Optional decay control
      decayControl    yes;
      kInf            \<far-field k value\>;
      omegaInf        \<far-field omega value\>;