Faster field copying

This removes the overhead of field copying.

`iris.load()` timings for a file containing 40 cubes (t: 22, z: 38:, y: 192, x: 155)

Code | Time to perform iris.load()
---|---
upstream/master | 6.2s (ignores vertical dimension; scaling by 38 gives 3m 48s)
pp-mo/lbc_levels | 4m 12s
This PR | 3m 19s