Ensure colorbar JSON is updated for model level data

With the ability to now use model level data in both LFRic and the UM we need to ensure consistent color bars for these types of plots. Given the granularity and changes between vertical levels we will need to agree a "set" of vertical levels that we will specify a narrow colorbar range for and then a wider range for when data is not in that set.

The issue does not name files, tests, or an entry point. Start by locating the colorbar JSON handling and the plotting paths for LFRic and UM model-level data, then clarify the agreed set of vertical levels and narrow versus wide ranges. Done means both data sources use consistent colorbars for the specified levels.

Written by the indexing model from the issue text.