Add the ability to automatically create metadata files when saving fieldsfiles

# ✨ Feature Request
Extend the fieldsfile saver so that the attributes of the cubes are saved as seperate metadata files rather than being discarded.
This is related to #32.

## Requirements
- If a cube with attributes is being saved as a fieldsfile, there should be the option to write out the attributes as separate `.attribute` (or other) files.

## Context
This will be used to preserve metadata that would otherwise be lost when saving data in a fieldsfile format.