ENH: mergePoints and patch gatherAndMerge improvements (#2402)

- when writing surface formats (eg, vtk, ensight etc) the sampled
  surfaces merge the faces/points originating from different
  processors into a single surface (ie, patch gatherAndMerge).

  Previous versions of mergePoints simply merged all points possible,
  which proves to be rather slow for larger meshes. This has now been
  modified to only consider boundary points, which reduces the number
  of points to consider. As part of this change, the reference point
  is now always equivalent to the min of the bounding box, which
  reduces the number of search loops. The merged points retain their
  original order.

- inplaceMergePoints version to simplify use and improve code
  robustness and efficiency.

ENH: make PrimitivePatch::boundaryPoints() less costly

- if edge addressing does not already exist, it will now simply walk
  the local face edges directly to define the boundary points.

  This avoids a rather large overhead of the full faceFaces,
  edgeFaces, faceEdges addressing.

  This operation is now more important since it is used in the revised
  patch gatherAndMerge.

ENH: topological merge for mesh-based surfaces in surfaceFieldValue