Provide support for running without OpenMP

### Contact Details

@mike-hobson

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

ticket:4695

### Brief description

It should be possible to build and run the LFRic codes without using OpenMP. However, at the moment, it is not.

### Further details of the issue.

There are currently calls to the OpenMP library (e.g. `omp_get_thread_num()` and `omp_get_max_threads()`) inside PSyKAl-lite code that are not protected by OpenMP sentinels - so when OpenMP is turned off, it results in undefined references.

Also, it would be good to includes a test in the test suite that checks that the no OpenMP build/run still works (but I guess this would have to be in the Apps test suite - so not part of this Core ticket).

Start by locating the PSyKAl-lite code that calls omp_get_thread_num() and omp_get_max_threads(), then inspect how OpenMP sentinels are handled elsewhere. The work is done when the core builds and runs without OpenMP and no undefined OpenMP references remain. The issue notes that a no-OpenMP runtime test would belong in the Apps test suite, not this core ticket.

Written by the indexing model from the issue text.