Further Refactor of excf_nl_9c

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

PR: #576
Umbrella: #106

### Brief description

This file has pushed the desire to avoid OMP syncronisation to the max, using multiple OMP master and barrier to effectively syncronise sections and fully span the parallel region over the whole file. While in an ideal world we want this, for porting source to other hardware and paralism methods, it might be worth diving the parallel sections up, and adjusting the source to be more simple.

The goal is to improve portability, but we may see a minor degrade in OMP performance. If it is very minor, than it will be acceptable and put to review. Otherwise we'll look at it again.

The removal of j in this file has already occurred, and so parallelism for LFRic on Mlian has improved. Genoa may perform differently.

### Further details of the issue.

_No response_