Difference plots for multi-level fields

[jfrost-mo]

What problem does your feature request solve?

With #1061 we have support for doing difference plots for surface fields, however not for multi-level fields like pressure.

Describe the solution you'd like

We should add some additional recipes to cater for multi-level difference plots. The existing operator should handle it, so we just need new recipes and include files.

Describe alternatives you've considered

None.