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 We have some notes on how to run the gpu-enabled MOM6, but is outdated.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