Use new last() method for List-type classes

- use first() method in some places as well where it helps clarity

- there are a few remaining cases: git grep 'size()-1]'

src/meshTools/triSurface/booleanOps/surfaceIntersection/surfaceIntersection.C

@@ -242,8 +242,7 @@ void Foam::surfaceIntersection::storeIntersection
 
             // Check whether perhaps degenerate
             const point& prevHit = allCutPoints[*iter];
-
-            const point& thisHit = allCutPoints[allCutPoints.size()-1];
+            const point& thisHit = allCutPoints.last();
 
             if (mag(prevHit - thisHit) < SMALL)
             {