Callbacks: file-writing (machine-readable files)

As a user/developer I would like the results of a scan to be preserved in a useful long-term scientific storage format.

Note: human-facing `.txt` files are being handled by https://github.com/ISISComputingGroup/ibex_bluesky_core/issues/7 , but that will not necessarily contain _all_ of the data, for example:
- Numbers will be formatted to the specified precision for human-readability
- Missing arrays
- etc

_This_ issue is to ensure the full/raw data is accessible in a scientist-facing way via standard file formats (e.g. HDF5 or similar).

`bluesky` already has support for this via `databroker` & `suitcase` - `suitcase` has plugins for various popular file formats including e.g. HDF5 that are suitable for long-term storage of bluesky scans data in an efficient machine-readable format that can handle all of bluesky's data types.