ENH: use nBoundaryFaces() in more places (less clutter, more clarity)

...yMesh/polyMeshTetDecomposition/polyMeshTetDecomposition.C

src/conversion/polyDualMesh/polyDualMesh.C

src/dynamicFvMesh/dynamicRefineFvMesh/dynamicRefineFvMesh.C

src/functionObjects/field/fluxSummary/fluxSummary.C

...Options/sources/derived/rotorDiskSource/rotorDiskSource.C

src/mesh/snappyHexMesh/meshRefinement/meshRefinement.C

src/mesh/snappyHexMesh/meshRefinement/meshRefinementBlock.C

src/mesh/snappyHexMesh/meshRefinement/meshRefinementRefine.C

src/meshTools/sets/faceSources/cellToFace/cellToFace.C