Support FieldsFile grid code 11

**Reviewer: cpelley**

Grid type code 11 is defined as: "Data on atmospheric uv points"

From the UM systems team:

> The phrase "Data on atmospheric uv points" for grid type 11 is, I guess, a little bit ambiguous, but means "data on the same common set of B-grid wind points, not the distinct u and v points of the C-grid"