Update to new open-source FIELD_API

[awnawab]

This PR updates the scc-field variant to use the new open-source FIELD_API. Notable changes are the use of the FieldGang feature to offload packed fields as a single contiguous entity, and the use of raw device pointers rather than host-mapped device pointers.

[reuterbal]

Thanks, this looks great and very much looking forward to switching to the open-source field-api repository! Currently, the NVHPC tests fail to compile field-api and it would be good to get this sorted.

[reuterbal]

Since this is effectively a global option in this form, we might want to simply put this at the top of the bundle.yml.

[awnawab]

Thanks for the review @reuterbal and for all the offline discussion. I am converting this to a draft since I will need a FIELD_API build-system update to get around building with acc=host and cuda (for the CLOUSC CI).

[awnawab]

Thanks again @reuterbal for all the help with this. The CI now builds the nvhpc runs with the -acc=gpu flag i.e. how we intend to build them for use. Because this inserts cuInit calls, we can't run these on the CI yet. Therefore, a separate nvhpc run is added with OpenACC explicitly disabled to test the loki and gpu variants (similar to the GNU CI runs).

[reuterbal]

I think that's a reasonable solution retaining coverage for some variants built with nvhpc and testing at least the build for the others. Many thanks for grinding through this!

[reuterbal]

As discussed: Merging now and let's have a 1-line change PR once the new FIELD_API version has been tagged!