Lookup dictionary for more friendly diagnostic names.

The names we get from LFRic are not particularly user friendly, and can be quite long which makes it difficult to display on plots. Some include _radiative_timestep in the title too, for example. There could be some value in having a dictionary that translates between these variables and human friendly ones, for plot titles etc. Huw recommended this one.

Start by locating where plot titles are generated and where LFRic variable names enter the plotting workflow. Review how names such as those containing _radiative_timestep are currently displayed, then define the mapping scope and verify that mapped titles appear correctly on plots.

Written by the indexing model from the issue text.