ENH: additional methods/operators for boundBox (related to #196)

- Constructor for bounding box of a single point.

- add(boundBox), add(point) ...
  -> Extend box to enclose the second box or point(s).

  Eg,
      bb.add(pt);
  vs.
      bb.min() = Foam::min(bb.min(), pt);
      bb.max() = Foam::max(bb.max(), pt);

Also works with other bounding boxes.
  Eg,
      bb.add(bb2);
      // OR
      bb += bb2;
  vs.
      bb.min() = Foam::min(bb.min(), bb2.min());
      bb.max() = Foam::max(bb.max(), bb2.max());

'+=' operator allows the reduction to be used in parallel
gather/scatter operations.

A global '+' operator is not currently needed.

Note: may be useful in the future to have a 'clear()' method
that resets to a zero-sized (inverted) box.

STYLE: make many bounding box constructors explicit

src/sampling/meshToMesh/meshToMeshParallelOps.C

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2012-2016 OpenFOAM Foundation
-     \\/     M anipulation  | Copyright (C) 2015 OpenCFD Ltd.
+     \\/     M anipulation  | Copyright (C) 2015-2017 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -177,14 +177,13 @@ Foam::autoPtr<Foam::mapDistribute> Foam::meshToMesh::calcProcMap
             const cell& c = cells[celli];
 
             // determine bounding box of tgt cell
-            boundBox cellBb(point::max, point::min);
+            boundBox cellBb(boundBox::invertedBox);
             forAll(c, facei)
             {
                 const face& f = faces[c[facei]];
                 forAll(f, fp)
                 {
-                    cellBb.min() = min(cellBb.min(), points[f[fp]]);
-                    cellBb.max() = max(cellBb.max(), points[f[fp]]);
+                    cellBb.add(points, f);
                 }
             }