Fix/filter fieldconvert

Issue/feature addressed

Fixes issue #340 (closed).

Furthermore, I have added the OutputStartTime capability to the FieldConvert filter, as was done for the FilterCheckpoint, see !1401 (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).