Fix data loading problems.

(Replaces old https://github.com/pp-mo/iris/pull/11)

Changes to FF loading specific to LBC files,  to make the data load properly.

All the tests are done in a single, inaccurately named `test_coords()` method. It'd be nicer if they were split out into multiple test methods. In which case you might want to avoid doing this load multiple times. But it's a pretty quick load, so you could just repeat it. Your call.