TurbulenceModels: Created a general base-class and selection mechanism for laminar stress models

Renamed the original 'laminar' model to 'Stokes' to indicate it is a
linear stress model supporting both Newtonian and non-Newtonian
viscosity.

This general framework will support linear, non-linear, visco-elastic
etc. laminar transport models.

For backward compatibility the 'Stokes' laminar stress model can be
selected either the original 'laminar' 'simulationType'
specification in turbulenceProperties:

    simulationType laminar;

or using the new more general 'laminarModel' specification:

    simulationType laminar;

    laminar
    {
        laminarModel        Stokes;
    }

which allows other laminar stress models to be selected.