Add new combined imputation and derived flag column

[ldavies99]

Summary

Add line at the end of the impute wrapper function to create a new combined imputation and derived flag column.

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

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[Jday7879]

This is a nice solution!
Is it a potential issue that if our imputation logic fails and we fail to impute, this would also be marked as a "d". Might be worth to specify "d" based on the "derived" column, and add a new letter to fill the final nans?

Happy to approve if this is something you have already checked :)

[Jday7879]

Nice work Luke! I didnt even clock that constrained questions would also have the d marker placed on them. this should solve that in cases when one has been imputed :)
(I'll sort the hooks now )