ENH: add blockMeshDict "mergeType" keyword (#1589)

- enumerated values are (points | topology) which can be optionally
  specified in the blockMeshDict. Default is 'topology'.

  If the command-line option `blockMesh -merge-points` is specified,
  this has absolute priority over any blockMeshDict entry.

STYLE: changed blockMesh "-blockTopology" option to "-write-obj"

- this is more specific to what it does. Potentially wish to add a
  "-write-vtk" option in the future.

TUT: adjust tutorials to use preferred or necessary merge strategies:

  * channel395DFSEM - topology
  * nozzleFlow2D - points
  * pipeCyclic - points