ENH: Added new function object to compute the Proudman acoustic power

Calculates the acoustic power due to the volume of isotropic turbulence
using Proudman's formula

The acoustic power \f$ P_A \f$ [W/m3] in terms of turbulence \f$ k \f$
and \f$ \epsilon \f$ is given as:

    \f[
        P_A = alpha_\epsilon \rho \epsilon M_t^5
    \f]

where \f$ alpha_\epsilon \f$ is a constant (0.1) and

    \f[
        M_t = \frac{\sqrt{2 k}}{a_0}
    \f]

with \f$ a_0 \f$ the speed of sound.  The acoustic power is also output in
dB using:

    \f[
        L_P = 10 \log \frac{P_A}{P_ref}
    \f]

where \f$ P_ref \f$ is a constant (1e-12 W/m3)

Usage
    Example of function object specification to calculate the Proudman acoustic
    power

    proudmanAcousticPower1
    {
        type        proudmanAcousticPower;
        libs        ("libfieldFunctionObjects.so");
        ...

        // Required additional entries for incompressible calculations
        rhoInf      1.225;
        aRef        340;
    }

    Where the entries comprise:
        Property     | Description                 | Required   | Default value
        type         | type name: proudmanAcousticPower         | yes        |
        rhoInf       | Freestream density for incompressible cases  | no |
        aRef         | Reference spped of sound for incompressible cases | no |
        alphaEps     | Model coefficient           | no         | 0.1

Note
- The freestream density and reference speed of sound are only necessary
  when a thermodynamics package is unavailable, typically for incompressible
  cases.