Update _stash_to_lfric.py for missing model level variables

With the ability to plot model level data we need to ensure that `_stash_to_lfric.py` is up-to-date and contains all relevant fields so that we can compare UM and LFRic.

Variables to check to see if there is a UM equivalent (and if they need to be added to `_stash_to_lfric.py` are

- cloud_droplet_number_concentration
- cloud_liquid_water_mixing_ratio
- divergence_of_wind
- eastward_wind
- m_r
- northward_wind
- potential_vorticity
- upward_heat_flux_in_air

Open `_stash_to_lfric.py` and inspect how existing fields are mapped for model-level data. Check each listed variable for a UM equivalent, add the relevant fields to the mapping, and verify that the resulting entries support comparison between UM and LFRic.

Written by the indexing model from the issue text.