STYLE: use edge::unitVec() for improved code clarity

applications/utilities/mesh/advanced/splitCells/splitCells.C

applications/utilities/surface/surfaceHookUp/surfaceHookUp.C

...OAM/meshes/primitiveMesh/PatchTools/PatchToolsSortEdges.C

...dynamicMesh/polyTopoChange/polyTopoChange/edgeCollapser.C

src/finiteArea/faMesh/faMeshDemandDrivenData.C

...esh/snappyHexMesh/refinementFeatures/refinementFeatures.C

src/meshTools/cellFeatures/cellFeatures.C

src/meshTools/meshTools/meshTools.C

...faces/searchableExtrudedCircle/searchableExtrudedCircle.C