dev: Use kwargs only in dataclasses

[thomaspinder]

Now that our minimum Python version is 3.10, we can make use of the kw_only argument in the dataclass constructor and remove the awkward None initialisation. To resolve this issue, the following steps would need to be taken

1. Decorate all dataclasses with kw_only=True
2. Remove all None argument defaults
3. Cleanup the corresponding __post_init__

[daniel-dodd]

Tagging #372 as closely related issue.

[github-actions[bot]]

This issue has been marked as stale because it has been open for 7 days with no activity.