Resolve "Trends diagnostics may not be scientifically correct in RK3" (branch_5.0)

Closes #487 (closed)

Tests

Regular checks

• Can this change be shown to produce expected impact (option activated)?
• Can this change be shown to have a null impact (option not activated)?
• Results of the required bit comparability tests been run: are there no differences when activating the development?
• Is this change expected to preserve all diagnostics?

  • If no, is reason for the change valid/understood?

    The trends diagnostics are disabled if activated, so the netCDF-based diagnostics will now be returned as missing values (the default if XIOS does not receive any model data).

Review

Assessments

• Is the proposed methodology now implemented?
• Are the code changes in agreement with the flowchart defined at preview step?
• Are the code changes in agreement with list of routines and variables as proposed at preview step?
  • If, not, are the discrepancies acceptable?
• Is the in-line documentation accurate and sufficient?
• Do the code changes comply with NEMO coding standards?
• Is the development documented with sufficient details for others to understand the impact of the change?
• Is the project doc (manual, guide, web, ...) now updated or completed following the proposed summary in preview section?