buoyantBoussinesq[SP]impleFoam: replaced by the more general buoyant[SP]impleFoam solvers

With the selection of the Boussinesq equation of state the general buoyancy
solvers buoyantSimpleFoam and buoyantPimpleFoam can be used instead of the
specialised Boussinesq solvers avoiding the need for special implementation of
thermal and pressure boundary conditions and providing support for radiation and
fvOptions which would not have been feasible or practical in the Boussinesq
solvers.

Other incompressible equations of state are also supported; for most gaseous
problems the incompressiblePerfectGas equation of state is likely to be more
accurate than the Boussinesq equation of state.

The buoyantBoussinesq[SP]impleFoam tutorials have been updated and moved to the
corresponding buoyant[SP]impleFoam directories.

src/thermophysicalModels/basic/rhoThermo/rhoThermos.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     | Website:  https://openfoam.org
-    \\  /    A nd           | Copyright (C) 2011-2018 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2019 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -410,6 +410,18 @@ makeThermos
     specie
 );
 
+makeThermos
+(
+    rhoThermo,
+    heRhoThermo,
+    pureMixture,
+    constTransport,
+    sensibleInternalEnergy,
+    eConstThermo,
+    Boussinesq,
+    specie
+);
+
 makeThermos
 (
     rhoThermo,