Propagate the changes from #6338 to apply to all remaining objects e.g. auxiliary coordinates, cell methods

See #6317 for more specifics

• Iris __init__.py
• helpers.py
• actions.py
• Tests
• Confirm this can fix known cases (Common agreement on loading CF non-compliant NetCDF files #5165)
  • Consider curating these examples into files ahead of this step
• Docstrings
• What's New