Updating Iris' handled grids to include grid 21

Grid 21 is being handled correctly by Iris, it should be on a P grid. The documentation isn't clear but this has been decided as the land sea mask used for the compressing is on unstaggered theta points.

Stash item m01s00i207 wasn't in the STASH to grid type mapping so was getting a warning.
With only a single test method I probably wouldn't bother with a `setUp`, but it won't stop me merging. However, please add a `tests.skip_data` decorator as this test relies on test data. If you keep the set up, you should decorate the class, if you don't just decorate the method.