Unify colourbars across models

[Sylviabohnenstengel]

Describe the bug

1. relative humidity has different colourbars for spatial plots between UM and LFRic.
   We need to check if this is the case for other variables.

2. Currently spatial plost show different ticks and scales between different models when plotting the same variable. We currently ensure the same colourbar within each model run, but do we need to fix this between different models as well? How would be broadcast that infromation across the workflow @jfrost-mo ?

How to reproduce

Steps to reproduce the behaviour:

Expected behaviour

Environment

• Version: [e.g. from cset --version]
• Browser (if UI issue): [e.g. Chrome, Firefox]

[Sylviabohnenstengel]

Branch: https://github.com/MetOffice/CSET/pull/new/1037_unify_colourbar