#554: Improve set up of fields on coarse grid in LFRic

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Currently, LFRic atmosphere contains a few fields (aerosols, nudging after #474) defined on a coarse mesh i.e. different mesh to 'dynamics'.
The setup and reation of these fields however, requires adding/ using bespoke arguments to field creation functions, eg. in *create_physics_prognostics*
```
call processor%apply(make_spec('ait_sol_om', main%aerosol, Wtheta, **coarse=coarse_rad_aerosol, &
coarse_mesh_name=mesh_name**, adv_coll=if_adv(advection_flag, adv%last_con), &
ckp=checkpoint_flag))
```
If there are another set of physics fields that move to a coarse mesh this may add further complexity to the definitions.

There is hence a need to design a robust and flexible way to handle the coarse mesh fields.

### Further details of the issue.

_No response_

Start by reading create_physics_prognostics and the field creation calls using coarse_rad_aerosol and coarse_mesh_name. Review the coarse-mesh changes associated with issue #474 and trace how coarse fields are currently created. Done means a documented, robust design is implemented for handling additional coarse-mesh physics fields without adding bespoke creation arguments.

Written by the indexing model from the issue text.