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.

[github-actions[bot]]

There has been no recent activity on this issue. To keep our issues log clean, we remove old and inactive issues. Please update to the latest version of GPJax and check if that resolves the issue. Let us know if that works for you by leaving a comment. This issue is now marked as stale and will be closed if no further activity occurs. If you believe that this is incorrect, please comment. Thank you!

[github-actions[bot]]

There has been no activity on this PR for some time. Therefore, we will be automatically closing the PR if no new activity occurs within the next seven days. Thank you for your contributions.