461 turnover analysis tool

[lhubbardONS]

Summary

Add your summary here - keep it brief, to the point, and in plain English.

Checklists

This pull request meets the following requirements:

• installable with all dependencies recorded
• runs without error
• follows PEP8 and project specific conventions
• appropriate use of comments, for example no descriptive comments
• functions documented using Numpy style docstings
• assumptions and decisions log considered and updated if appropriate
• unit tests have been updated to cover essential functionality for a reasonable range of inputs and conditions
• other forms of testing such as end-to-end and user-interface testing have been considered and updated as required
• tests suite passes (locally as a minimum)
• peer reviewed with review recorded

If you feel some of these conditions do not apply for this pull request, please
add a comment to explain why.

[KateyMatthews]

Luke H went through each step of this function with me - happy to approve :)