Resolve "Undefined values in FCT tracer advection scheme when using MLF and 'accurate' adaptive-implicit vertical advection algorithm"

Closes #499 (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?

Other testing

SETTE does not reproduce this bug since none of the tests use nn_fct_imp=2. A 10-day ORCA2 run was therefore used to reproduce the issue and verify that the fix both resolved this and did not change 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?