Check for nans when saving fieldsfiles

# ✨ Feature Request
Some error checking should take place when saving fieldsfiles, so that an error is produced if nans are present, as ancillary fieldsfiles should not contains nans.

## Requirements
- When saving data as a fieldsfile (save.ancil), the data to be saved should be checked to ensure it contains no nans.

## Context
- Issues were prviously found where nans were present in rose stem data outputs and should not have been. This should be prevented from happening again by throwing an error.

Start by locating the save.ancil implementation and the code path that writes fieldsfiles. Trace how the data is passed into the writer, then identify the existing saving tests if available. Done means saving data containing NaNs raises an error, while valid ancillary fieldsfiles continue to save successfully.

Written by the indexing model from the issue text.