Resolve "refactor zdf_drg and fix issue with rCdU_top when using linear drag"

Closes #383 (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?
  • If some differences appear, is reason for the change valid/understood?
  • If some differences appear, is the impact as expected on model configurations?
• Is this change expected to preserve all diagnostics?
  • If no, is reason for the change valid/understood?
• Are there significant changes in run time/memory?

SETTE passes, but it doesn't cover the specific configuration settings (key_si3 + key_isf + ln_isfcav + ln_lin + ln_drgice_imp) that would recreate this issue.

Other tests

Testing was performed using 10-day runs of ORCA2_ICE_PISCES with passive tracers turned off. The restartability failures caused by this issue have been addressed, which causes an expected change in results.

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?