added in a vertical rule for surface fields

[ESadek-MO]

🚀 Pull Request

Closes #3820.

[pp-mo]

Note about the currently failing tests ...

These PP save tests are checked by comparing a string representation of all the resulting PP fields against one stored in a text file under iris/tests/results/cube_to_pp.
This is done with the "cube_save_test" method supplied here.

So, you should just delete all those files, and run all tests which will re-make them. Then commit the resulting changes, which can then be reviewed.

NOTE: for no very good reason AFAICT, this check is implemented separately from the other compare-to-stored-file tests of this nature, and does NOT require IRIS_TEST_CREATE_MISSING=1 to be set (which was probably just an oversight IMHO).