ENH: proudmanAcousticPower - extended to operate on mean turbulence fields

Example using mean turbulence fields (mean fields should be available e.g. from
a fieldAverage function object)

    proudmanAcousticPower1
    {
        // Mandatory entries (unmodifiable)
        type        proudmanAcousticPower;
        libs        (fieldFunctionObjects);

        ...

        // Turbulence field names (if not retrieved from the turb model)
        k           kMean;
        epsilon     epsilonMean;
        omega       none; // omegaMean
    }