Provides run-time selection of buoyancy sources for compressible solvers

Replaces the built-in buoyancy sources in XiFoam, reactingFoam and
rhoReactingFoam.

e.g. in constant/fvOptions specify

momentumSource
{
    type            buoyancyForce;

    buoyancyForceCoeffs
    {
        fieldNames      (U);
    }
}

and optionally specify the buoyancy energy source in the enthalpy
equation:

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (h);
    }
}

or internal energy equation

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (e);
    }
}