equationOfState/Boussinesq: New equation of state for the Boussinesq approximation for buoyant flows

Description
    Incompressible gas equation of state using the Boussinesq approximation for
    the density as a function of temperature only:

    \verbatim
        rho = rho0*(1 - beta*(T - T0))
    \endverbatim

To be used with the buoyantPimpleFoam and buoyantSimpleFoam solvers as
an alternative to using buoyantBoussinesqPimpleFoam or
buoyantBoussinesqSimpleFoam, providing consistency with all other
solvers and utilities using the thermodynamics package in OpenFOAM.