ENH: boundBox improvements (#2609)

- construct boundBox from Pair<point> of min/max limits,
  make sortable

- additional bounding box intersections (linePointRef), add noexcept

- templated access for boundBox hex-corners
  (used to avoid temporary point field).
  Eg, unrolled plane/bound-box intersection with early exit

- bounding box grow() to expand box by absolute amounts
  Eg,

      bb.grow(ROOTVSMALL);   // Or: bb.grow(point::uniform(ROOTVSMALL));
  vs
      bb.min() -= point::uniform(ROOTVSMALL);
      bb.max() += point::uniform(ROOTVSMALL);

- treeBoundBox bounding box extend with two or three parameters.
  The three parameter version includes grow(...) for reduced writing.
  Eg,

      bb = bb.extend(rndGen, 1e-4, ROOTVSMALL);

  vs
      bb = bb.extend(rndGen, 1e-4);
      bb.min() -= point::uniform(ROOTVSMALL);
      bb.max() += point::uniform(ROOTVSMALL);

  This also permits use as const variables or parameter passing.
  Eg,

      const treeBoundBox bb
      (
          treeBoundBox(some_points).extend(rndGen, 1e-4, ROOTVSMALL)
      );

applications/test/findSphereFeatureEdges-octree/Test-findSphereFeatureEdges-octree.C

@@ -64,13 +64,8 @@ int main(int argc, char *argv[])
 
     // Slightly extended bb. Slightly off-centred just so on symmetric
     // geometry there are less face/edge aligned items.
-    treeBoundBox bb
-    (
-        efem.points()
-    );
-
-    bb.min() -= point::uniform(ROOTVSMALL);
-    bb.max() += point::uniform(ROOTVSMALL);
+    treeBoundBox bb(efem.points());
+    bb.grow(ROOTVSMALL);
 
     labelList allEdges(identity(efem.edges().size()));