297 omp sentinels

[hiker]

Adds some support for OMP sentinels (#297), which are required for compiling lfric_atm.

The next release of fparser adds support for sentinels to fparser, so this change can then be removed again (#todo in place to remind us, so I would also leave #297 open), we only need to trigger this behaviour in fparser (which means it will not only work for declaration statements). We need this patch for now to continue LFRic compilation (but the source code in question is UM, so I added label UM, and not LFRic).

[MatthewHambley]

What's the timescale for the fParser fix and the need for directive support in Fab?

[hiker]

Ready for next review.

[hiker]

What's the timescale for the fParser fix and the need for directive support in Fab?

I've just submitted my last additional PR that I wanted for the next release, and there seems to be only one issue that might need to be handled before the next release. An fparser release is usually done very quickly, so I would hope (depending on priorities) to get this in maybe a month?

I originally intended not to push this PR to the official repo at all, but we had to integrate all these changes into our PRs (it became very messy to keep track and merging things), so it simplifies things a bit for us. E.g. #301 has this patch and it's not easy to remove it (conflicts).

[MatthewHambley]

I've raised #327 to remove these changes when they are no longer needed.