feat: streamline notebooks

[ESKYoung]

Summary

Clean up the notebook directory by renaming it to exploratory, and:

• organising exploratory tasks
• creating a exploratory Python environment constrained by the underlying Poetry environment
• adding example exploratory packages, tests, and documentation.

It's not ideal to have both Poetry, and pip working in the same repository. Nor is it ideal that the exploratory environment modifies the underlying Python environment used by Poetry, but at least the Poetry configuration is unaffected.

Checklists

I/we (contributor) confirm that the code, and analyses in this Pull Request (developments) meets the following requirements:

• code runs
• developments are ethical, and secure
• contributor has made proportionate checks that the developments are correct
• minimum usable documentation is written in plain English in the docs folder
• all pre-commit hooks pass
• test suite passes
• code coverage is maintained, or increased

Additional comments