ENH: add enthalpy sub-looping for chtMultiRegion* solvers

...vers/heatTransfer/chtMultiRegionFoam/chtMultiRegionFoam.C

...nFoam/chtMultiRegionSimpleFoam/chtMultiRegionSimpleFoam.C

...oam/chtMultiRegionSimpleFoam/fluid/setRegionFluidFields.H

...ltiRegionFoam/chtMultiRegionSimpleFoam/solid/solveSolid.H

...heatTransfer/chtMultiRegionFoam/fluid/createFluidFields.H

...tTransfer/chtMultiRegionFoam/fluid/setRegionFluidFields.H

...heatTransfer/chtMultiRegionFoam/solid/createSolidMeshes.H

...olvers/heatTransfer/chtMultiRegionFoam/solid/solveSolid.H

src/functionObjects/field/externalCoupled/externalCoupled.C

src/functionObjects/field/externalCoupled/externalCoupled.H

...d/externalCoupledMixed/externalCoupledMixedFvPatchField.H

...Mixed/externalCoupledTemperatureMixedFvPatchScalarField.H

...ionModels/regionModel/regionProperties/regionProperties.H