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