Copying one intrinsic Fortran numeric precision field to another

Currently there is no support for copying one intrinsic Fortran numerical precision field to another (e.g., from r32 to r64).

There are currently 4 PSyKAl-lite routines which exist because this functionality doesn't exist. These are:

- `invoke_copy_field_32_64`
- `invoke_copy_field_64_32`
- `invoke_copy_field_32_32`
- `invoke_copy_field_64_64`

and are found in LFRic core `components/science/source/psy/sci_psykal_builtin_light_mod`. These are all called by `components/science/source/algorithm/copy_field_alg_mod.f90` in LFRic core.

The related PSyclone Issue is [#2674](https://github.com/stfc/PSyclone/issues/2674).

Start by reading components/science/source/psy/sci_psykal_builtin_light_mod and components/science/source/algorithm/copy_field_alg_mod.f90 to understand the four existing precision-specific routines and their callers. Review PSyclone issue #2674 for related context. Done means intrinsic Fortran fields can be copied between numerical precisions without relying on those four separate routines.

Written by the indexing model from the issue text.