MAJOR FIX: using background pressure field for t=2 and not t=1 makes massive difference.
MAJOR FIX: using background pressure field for t=2 and not t=1 makes massive difference.

1. Doing this improves comparision against prsgrd diagnostic by a lot.

dcherian/eddyshelf,dcherian/eddyshelf,dcherian/eddyshelf