rhoThermo: Renamed thermo:rho -> rho

The thermodynamic density field is now named "rho" by default and only renamed
"thermo:rho" by solvers that create and maintain a separate continuity density
field which is named "rho".  This change significantly simplifies and
standardises the specification of schemes and boundary conditions requiring
density as it is now always named "rho" or "rho.<phase>" unless under some very
unusual circumstances the thermodynamic rather than continuity density is
required for a solver maintaining both.

The advantage of this change is particularly noticeable for multiphase
simulations in which each phase has its own density now named "rho.<phase>"
rather than "thermo:rho.<phase>" as separate phase continuity density fields are
not required so for multiphaseEulerFoam the scheme specification:

    "div\(alphaRhoPhi.*,\(p\|thermo:rho.*\)\)" Gauss limitedLinear 1;

is now written:

    "div\(alphaRhoPhi.*,\(p\|rho.*\)\)" Gauss limitedLinear 1;

applications/solvers/modules/fluid/compressibleVoF/compressibleTwoPhaseMixture/compressibleTwoPhaseMixture.C

@@ -79,7 +79,7 @@ Foam::compressibleTwoPhaseMixture::compressibleTwoPhaseMixture
     (
         IOobject
         (
-            "thermo:rho",
+            "rho",
             U.mesh().time().timeName(),
             U.mesh(),
             IOobject::NO_READ,

applications/solvers/modules/fluid/compressibleVoF/fvModels/VoFCavitation/VoFCavitation.C

@@ -130,7 +130,7 @@ void Foam::fv::compressible::VoFCavitation::addSup
     if (fieldName == "p_rgh")
     {
         const volScalarField::Internal& rho =
-            mesh().lookupObject<volScalarField>("thermo:rho");
+            mesh().lookupObject<volScalarField>("rho");
 
         const volScalarField::Internal& gh =
             mesh().lookupObject<volScalarField>("gh");

applications/solvers/modules/fluid/isothermalFluid/isothermalFluid.C

@@ -79,7 +79,7 @@ Foam::solvers::isothermalFluid::isothermalFluid
             IOobject::READ_IF_PRESENT,
             IOobject::AUTO_WRITE
         ),
-        thermo.rho()
+        thermo.renameRho()
     ),
 
     dpdt