498 sampled_flag is providing flag for census groups

[AntonZogk]

*Check if calibration group is cencus
*Add unit test, test data
*Remove correct_values function,test,test data
*Implement is_cencus in apply estimation

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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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[robertswh]

We could do with a test for apply estimation, I'll add to backlog. There doesn't seem to be an obvious home for the is_census() function, but where it is now is fine. We can re-assess at a later date.