ENH: add geometric decomposition constraint (issue #921)

- geometric decomposition constraints may be used to prevent the
  decomposition of regions of the mesh.

  The geometric constraint is applied according to the face centres,
  which define the connectivity between cells.

  Specified in decomposeParDict

    constraints
    {
        geometric
        {
            type    geometric;

            geometry
            {
                box1
                {
                    type    box;
                    min     (-10 -10 -10);
                    max     (1 1 1);
                }

                ball1
                {
                    type    sphere;
                    origin  (-2 -2 1);
                    radius  1;
                }
            }
        }
    }

ENH: add 'grow' option for geometric decomposition constraint (issue #921)

- the 'grow' option includes an additional check to include cell faces
  for any cell that already has two or more of its faces "unblocked".
  This could indicate a connection over a corner, but does not distinguish
  between connectivity introduced by the constraint and the connectivity
  defined by other constraints.