externalWallHeatFluxTemperature: Added optional support for radiative flux to the outside

By specifying the optional outside surface emissivity radiative heat transfer to
the ambient conditions is enabled.  The far-field is assumed to have an
emissivity of 1 but this could be made an optional input in the future if
needed.

Relaxation of the surface temperature is now provided via the optional
"relaxation" which aids stability of steady-state runs with strong radiative
coupling to the boundary.