Lazy iris.coords.Coord.cell method

[bouweandela]

✨ Feature Request

At the moment, the iris.coords.Coord.cell method has the side effect of loading all the coordinate data from disk into memory.

Motivation

It would be more efficient if only the requested coordinate point would be loaded. If users want to load all data, e.g. because they plan to access many cells, they can do so themselves.