(closes #3243) Unify and document nemo scritps

[sergisiso]

• Unifies omp_gpu_trans.py omp_cpu_trans.py and acc_loops_trans.py
• Adds a new README (https://github.com/stfc/PSyclone/tree/single_nemo_script/examples/nemo/scripts)
• Switch one of the NEMOv5 ORCA test from OpenMP to OpenACC loops to have coverage of both
• Improve performance of NEMOv5 (NEMOv4 got a regression needed to get full reproducibility)
• Require full reproducibility of remaining nemo (loop) tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (91d493f) to head (69ad36f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3244   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files         378      378           
  Lines       53764    53778   +14     
=======================================
+ Hits        53742    53756   +14     
  Misses         22       22           

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[sergisiso]

@LonelyCat124 This is now ready for another look, (you will see that I submited a few integration tests that failed, so I force-pused a removal of these last commits), the integration of the current commit is successful.

The slowdown for OpenACC loops is necessary to have full numberical reproducibility, before it was tested with a tolerance, but this can hide small issues, so I prefer to be strict on this even if we lose performance.

[LonelyCat124]

@sergisiso Can you fix up the conflicts that have appeared and I'll review this again - I can't view them in the web editor annoyingly as I suspect that may end up being copyright changes but I'm not sure.