porosityModels::solidification: Added optional phase-fraction for VoF solvers etc.

Description
    Simple solidification porosity model

    This is a simple approximation to solidification where the solid phase
    is represented as a porous blockage with the drag-coefficient evaluated from

        \f[
            S = - \alpha \rho D(T) U
        \f]

    where
    \vartable
        \alpha  | Optional phase-fraction of solidifying phase
        D(T)    | User-defined drag-coefficient as function of temperature
    \endvartable

    Note that the latent heat of solidification is not included and the
    temperature is unchanged by the modelled change of phase.

    Example of the solidification model specification:
    \verbatim
        type            solidification;

        solidificationCoeffs
        {
            // Solidify between 330K and 330.5K
            D table
            (
                (330.0     10000) // Solid below 330K
                (330.5     0)     // Liquid above 330.5K
            );

            // Optional phase-fraction of solidifying phase
            alpha alpha.liquid;

            // Solidification porosity is isotropic
            // use the global coordinate system
            coordinateSystem
            {
                type    cartesian;
                origin  (0 0 0);
                coordinateRotation
                {
                    type    axesRotation;
                    e1      (1 0 0);
                    e2      (0 1 0);
                }
            }
        }
    \endverbatim