externalWallHeatFluxTemperatureFvPatchScalarField: Added "power" heat source option

by combining with and rationalizing functionality from
turbulentHeatFluxTemperatureFvPatchScalarField.
externalWallHeatFluxTemperatureFvPatchScalarField now replaces
turbulentHeatFluxTemperatureFvPatchScalarField which is no longer needed and has
been removed.

Description
    This boundary condition applies a heat flux condition to temperature
    on an external wall in one of three modes:

      - fixed power: supply Q
      - fixed heat flux: supply q
      - fixed heat transfer coefficient: supply h and Ta

    where:
    \vartable
        Q  | Power [W]
        q  | Heat flux [W/m^2]
        h  | Heat transfer coefficient [W/m^2/K]
        Ta | Ambient temperature [K]
    \endvartable

    For heat transfer coefficient mode optional thin thermal layer resistances
    can be specified through thicknessLayers and kappaLayers entries.

    The thermal conductivity \c kappa can either be retrieved from various
    possible sources, as detailed in the class temperatureCoupledBase.

Usage
    \table
    Property     | Description                 | Required | Default value
    mode         | 'power', 'flux' or 'coefficient' | yes |
    Q            | Power [W]                   | for mode 'power'     |
    q            | Heat flux [W/m^2]           | for mode 'flux'     |
    h            | Heat transfer coefficient [W/m^2/K] | for mode 'coefficent' |
    Ta           | Ambient temperature [K]     | for mode 'coefficient' |
    thicknessLayers | Layer thicknesses [m] | no |
    kappaLayers  | Layer thermal conductivities [W/m/K] | no |
    qr           | Name of the radiative field | no | none
    qrRelaxation | Relaxation factor for radiative field | no | 1
    kappaMethod  | Inherited from temperatureCoupledBase | inherited |
    kappa        | Inherited from temperatureCoupledBase | inherited |
    \endtable

    Example of the boundary condition specification:
    \verbatim
    <patchName>
    {
        type            externalWallHeatFluxTemperature;

        mode            coefficient;

        Ta              uniform 300.0;
        h               uniform 10.0;
        thicknessLayers (0.1 0.2 0.3 0.4);
        kappaLayers     (1 2 3 4);

        kappaMethod     fluidThermo;

        value           $internalField;
    }
    \endverbatim