Add parallel-in-time capabilities to FieldConvert

Issue/feature addressed

Add parallel-in-time capabilities to FieldConvert. This MR was split from !1457 (merged)

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).