Halo may not have full width at corners

[mark-petersen]

In the ocean core, there are bit-for-bit differences between one step runs with different block partitions. Adding extra halo updates fixes it, but I suspect that in a few places the halo is actually not three cells wide, as specified, but two cells wide.

[mark-petersen]

For a single step, the error shows up as the difference between blocks, and it occurs at corners of the block:

[matthewhoffman]

From Developers Guide p.18 here http://mpas-dev.github.io/

Cells make up the primary mesh within MPAS. Halos for cells are described below:

• 0-Halo: All "owned" cells;

• 1-Halo: All cells that border 0-Halo cells but are not 0-Halo cells;

• 2-Halo: All cells that border 1-Halo cells but do not belong to a lesser
  halo;

• 3-Halo: All cells that border 2-Halo cells but do not belong to a lesser
  halo;

Cells can have an arbitrary number of halos, in which case this incremental
definition continues until the last halo layer.

Edges and Vertices have a slightly different definition and always include
an additional layer.

• 0-Halo: All "owned" edges/vertices;

• 1-Halo: All edges/vertices that border 0-Halo cells that are not 0-Halo
  edges/vertices;

• 2-Halo: All edges/vertices that border 1-Halo cells that are not 1-Halo
  edges/vertices;

• 3-Halo: All edges/vertices that border 2-halo cells that are not 2-Halo
  edges/vertices;