Stochastic physics checkpointing broken for coupled models

### Version

main

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

MetOffice#383

### What happened?

The LFRic coupled climate model runs with stochastic physics and checkpointing. However at vn3.2 it fails with the following error in xios_server_000.err:

`> Error [void CGrid::computeClientIndex()] : In file '/home/users/harry.shepherd/cylc-run/infrastructure_suite_deploy_exab_r5889/run1/share/xios/src/node/grid.cpp', line 735 -> Number of local index on client is different from number of received global indexRank of sent client 0.Number of local index 104. Number of received global index 119. `

This was probably caused by the addition of pull request MetOffice#383 and was not picked up by the rose-stem tests as the lfric_coupled rose-stem test does not have either stochastic physics and checkpointing (you need both of these to cause the failure). I have recreated the failure by turning these both on in the following branch:

https://github.com/DanCopsey/lfric_apps/tree/stochasic_checkpoint_rose_stem

and running the lfric_coupled rose-stem test.

My proposal it that this branch goes onto the trunk alongside whatever code change is needed to get it to work.

### Relevant log output

```shell
> Error [void CGrid::computeClientIndex()] : In file '/home/users/harry.shepherd/cylc-run/infrastructure_suite_deploy_exab_r5889/run1/share/xios/src/node/grid.cpp', line 735 -> Number of local index on client is different from number of received global indexRank of sent client 0.Number of local index 104. Number of received global index 119.
```