Enable reading of sea-ice ancils on categories

[timgraham-Met]

Are there any linked Issues or Pull Requests?

No response

Brief description

In coupled NWP, runs, we use the OSTIA SST and sea-ice ancils over lakes that are too small to be resolved by NEMO. As the coupled model uses multi-category sea-ice some modifications are needed to the reading of sea-ice ancils to allow them to be read on categories. Further, some changes are needed to the logic as previously it was assumed that coupled models would not need to read sea-ice ancils.

Further details of the issue.

No response

[timgraham-Met]

In developing this change, I've noticed that sst_source is trigger ignored when ancil_option is set to anything other than 'fixed'. However, the variable sst_source is used at multiple points in the code so I think this should be removed. As the code is currently working, I'd prefer to get it merged and open a new ticket to consider whether this should be changed.

[timgraham-Met]

Developer tests all pass with no changes to results. Only task failure is in metadata check because metadata for lfric atm in coupled task still points to vn3.0 and I have changed HEAD.