Support submodules load instr settings

Explanation of changes

@adamorangeqs , this should closes #307 (closed) .

MR contains two changes

• Recovering instrument settings for submodules works now.
• Calling set_datadir(None) now sets the data directory to the default datadir as documented.

Motivation of changes

The first change is critical for #307 (closed) . The second bug was found while writing the tests.

---

Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (main for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• CHANGELOG.rst and AUTHORS.rst have been updated (when applicable).
• CI pipelines pass
  • pre-commit run --all-files --hook-stage commit passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds successfully (CI and readthedocs),
  • windows tests pass (manually triggered by maintainers before merging).

---

For reference, the issues workflow is described in the contribution guidelines.