Fix for pressure coordinate name variants plus enforce units to be hPa

[jwarner8]

LFRic has used another pressure coordinate name 'pressure_levels', plus there were some issues where domain pressure level spatial plots didn't work (and you wouldn't be able to diff UM and LFRic), given UM units were hPa and LFRic was Pa (and so the levels argument could not be used across models).

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[github-actions]

[jwarner8]

Anne comment on why 'pressure_levels' is used: "It was just what was used in the rose-stem lfric output file and is also consistent with the global lfric suite. So not particularly chosen by us. If you can work with pressure_levels, then I'd be in favour of going with that, as I'm keen to be as consistent with other practices as possible."

[jfrost-mo]

Approach looks sensible, though there are a few coding things to improve.

[jfrost-mo]

Looks good to me.