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)
      );

src/parallel/distributed/distributedTriSurfaceMesh/distributedTriSurfaceMesh.C

@@ -356,7 +356,7 @@ bool Foam::distributedTriSurfaceMesh::read()
         //outsideVolType_ = volumeType::UNKNOWN;
         //if (surfaceClosed_)
         //{
-        //    point outsidePt(localBb.max()+localBb.midpoint());
+        //    point outsidePt(localBb.max()+localBb.centre());
         //    List<volumeType> outsideVolTypes;
         //    triSurfaceMesh::getVolumeType
         //    (
@@ -1539,7 +1539,7 @@ void Foam::distributedTriSurfaceMesh::collectLeafMids
             // No data in this octant. Set type for octant acc. to the mid
             // of its bounding box.
             const treeBoundBox subBb = nod.bb_.subBbox(octant);
-            midPoints.append(subBb.midpoint());
+            midPoints.append(subBb.centre());
         }
     }
 }