(Major) refactor of XIOS interface to make use of external fields

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Now that we have the concept of "external fields" they seem like a strong possible solution to XIOS interfacing. Not surprising as interfacing with 3rd parties which use different field representations is what they are designed for.

This would represent a considerable amount of work as it turns I/O inside out. Instead of it being an aspect of the field objects, it becomes a utility to which field objects are brought.

Advantages are likely to include reducing the amount of code in the field object and removing the function pointers used to handle I/O.

### Further details of the issue.

_No response_

No files, tests, or entry points are named. Start by locating the existing XIOS interface and external-field implementation, then map how field objects currently handle I/O and use function pointers. Done means the interface has been refactored around external fields while preserving required I/O behavior.

Written by the indexing model from the issue text.