functionObjects::scalarTransport: Added support for optional laminar and...

functionObjects::scalarTransport: Added support for optional laminar and turbulent diffusion coefficients

Description
    Evolves a passive scalar transport equation.

    - To specify the field name set the \c field entry
    - To employ the same numerical schemes as another field set
      the \c schemesField entry,
    - A constant diffusivity may be specified with the \c D entry,

    - Alternatively if a turbulence model is available a turbulent diffusivity
      may be constructed from the laminar and turbulent viscosities using the
      optional diffusivity coefficients \c alphaD and \c alphaDt (which default
      to 1):
      \verbatim
          D = alphaD*nu + alphaDt*nut
      \endverbatim

Resolves feature request https://bugs.openfoam.org/view.php?id=2453