This fixes #143:

The original logic was hopelessly confused about the order in which it tackled problems. It now follows a much clearer system:

• Clears out incorrect naming of 'standard' sheets.
• Looks for undocumented data worksheets.
• Checks each provided data worksheet to make sure it is present and that any linked external file is valid.

I've also tacked on the removal of pytest-flake8: running flake8 during pytest needlessly duplicates the pre-commit hook and the pytest-flake8 package relies on a seriously outdated version of flake8 (4.0.1 versus current 7.0.0). Our pre-commit hook has been auto-updated to 7.0.0 so retaining this in pytest is both pointless and divergent.