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.
Do you mean just the line "del ffv" or the whole test?