GENIE Gridspec Diagrams Tile

A grid tile is a contiguous area of grid cells distinct in some way from other such areas that together make up a grid mosaic. For instance, a tile may represent one side of cubed sphere, or an independently discretised region of an ocean grid that covers a particular body of water. A grid tile is composed of cells, which may be specified in different ways depending on their degree or regularity or uniformity:


 * Cells may be specified explicitly using sub-types of "cell"; the only such type available currently is the unstructured cell type.


 * Cells may be specified implicitly using a combination of the "regular" and "uniform" properties, and a horizontal coordinate system.

Vertical levels are specified separately from the horizontal grid -- currently this is done using a vertical coordinate system.