ENH: proudmanAcousticPower - extended to operate on mean turbulence fields

The proudmanAcousticPower function object has been updated with the option to operate on user-supplied 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
}