Support for TiledNdArray objects which are subsets of a larger Tileset

To get the full caching benefits that Tilesets provide it would be useful if TiledNdArray objects could describe a subset of the tiles generated for larger areas, this would allow reuse of existing tiles by CoverageJSON responses for sub regions of larger datasets.

One way this could be acheived would be to add an optional origin member to the TiledNdArray object, this would be an integer array of the same shape as the tileShape member which would describe the origin index values for a TileSet. The values provided by the origin member could then be used by CoverageJSON clients to adjust the values provided by the tileShape member so that they described a subset of the larger tileset. If an origin member was not specified a client would treat the origin values for all dimensions to be zero.

Start by locating the TiledNdArray and tileShape definitions in the CoverageJSON specification. Review how CoverageJSON clients interpret tileShape and determine how an optional origin member would describe a subset of a larger Tileset. Done means the specification clearly defines origin, its shape and default zero values, plus the resulting client behavior.