Implement OpenMP offload for do group update

[edoyango]

Description
This adds the necessary code to make mpp_do_group_update work with arrays that are managed by NVIDIA's OpenMP offload runtime. This attempts to be minimally disruptive in that non-nvidia compilers will see the same behaviour as previously by adding macros around the relevant openmp directives.

Fixes #1771

How Has This Been Tested?
The OpenMP offload capability is currently tested on the "double gyre" case in MOM6-examples using the nvfortran compiler and a cuda-aware openmpi. We have some notes on how to run the gpu-enabled MOM6, but is outdated.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes

[nikizadehgfdl]

How are these two implementations equivalent? Is new idx = old pos always?

[edoyango]

Yes, they're equivalent. For any iteration, idx = pos + (k-1)*nj*ni + (j-js)*ni + (i-is) + 1 produces the same value that pos would have had at that point. The formula accounts for all the iterations that would have occurred in the nested loops up to that (i,j,k) position.

The reason for the change is that each nested iteration is now independent and can be performed in parallel.