Verify (and correct if necessary) the bounds of CBMTILE

[prushforth]

As written, they don't work as a gridset definition in GeoWebCache. For GeoWebCache, you need to use the CBMTILE defined origin as the min-x and max-y values and then specify some large x and negative y values for the max-x and min-y values, respectively. It would be nice to have something sensible in the specification for bounds, so that the bounds might be portable across implementations. It may be just a quirk of how GeoWebCache defines the origin of a gridset (which it derives from the bounds you enter -- doesn't use an explicit origin concept).