ENH: improve edge access for face/triFace

- additional rcEdge(), rcEdges() methods for reverse order walk

- accept generic edge() method as alternative to faceEdge() for
  single edge retrieval.

- edge() method with points -> returns the vector

- reduce the number of operations in edgeDirection methods

DEFEATURE: remove longestEdge global function

- deprecated and replaced by face::longestEdge() method (2017-04)

src/sampling/surface/cutting/cuttingSurfaceBaseTemplates.C

@@ -134,7 +134,7 @@ void Foam::cuttingSurfaceBase::walkCellCuts
 
             forAll(f, fp)
             {
-                edge e(f.faceEdge(fp));
+                edge e(f.edge(fp));
 
                 // Action #1: Orient edge (+ve gradient) and detect intersect
                 if (!edgeOrientIntersect(e))