Routine that writes atm dag to file should only run on one rank

[bartgol]

It is currently run by all ranks, each trying to write the same file. It is buggy at best.

Also, we should plot the "global" layout of a var, not the "on-rank" one. That is, if a layout has the COL label, we should make sure its extent matches the global number of cols, not the local (on-rank) one.