No isothermal profile option for planar domain initialisation

### Version

main

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

_No response_

### What happened?

I am using LFRic v2.2 in a planar domain CRM setup and am trying to use the isothermal test initialisation profile cases. Currently, when selecting the test_isot_dry_atm case, the logic in [reference_profile_mod.f90](https://github.com/MetOffice/lfric_apps/blob/7bcc57d6ce3d4d19d559d74832ef4a879088ab89/science/gungho/source/kernel/diagnostics/reference_profile_mod.F90#L136C1-L138C27) fails through to the 'default' case:
```
case default
theta_s = theta_surf
exner_s = exner_surf
```

Additionally, the [test_isot_atm case is currently using the same profile as test_gravity_wave](https://github.com/MetOffice/lfric_apps/blob/7bcc57d6ce3d4d19d559d74832ef4a879088ab89/science/gungho/source/kernel/diagnostics/reference_profile_mod.F90#L107-L112), which does not seem to be strictly isothermal.

It would be helpful if these isothermal test cases could be implemented for the planar domain.

### Relevant log output

```shell

```

Start in science/gungho/source/kernel/diagnostics/reference_profile_mod.F90, examining the planar-domain profile selection around the referenced isothermal and gravity-wave cases. Trace how test_isot_dry_atm and test_isot_atm are selected and compare their profiles with the expected isothermal setup. Done means both isothermal test cases initialize the intended planar profiles rather than falling through to default or reusing the gravity-wave profile.

Written by the indexing model from the issue text.