Cells stored as JSON are not precise

[alexgleith]

Loading one of the cells, for example, results in a loaded grid of y: 3201 x: 3200, which has an extra y.

We should move to using the JSON as a guide, and the actual grid definition from odc.geo...

This way we get nice things like this:

[jessjaco]

I generally support this. I do tend to use derived grid products which can take a while to produce, such as

• Only grid cells which intersect land
• Only the land within grid cells which intersect land

Making these on the fly, at least if running across the entire Pacific can take a while and I would not want to do it more than once if I could help it. On the other hand, if we load a cell, then do the intersection on only that cell, it might be OK.