420 flag for winsorisation

[sarahcollyer]

Summary

Added function to flag rows for winsorisation.
Works by multiplying the a weight and g weight columns together, then flagging any rows where the result is =<1.

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.

other forms of testing such as end-to-end and user-interface testing have been considered and updated as required - not considered yet as we have only just begun coding this method

[sarahcollyer]

Have also split test data into flag_data.csv which only contains the columns needed for this function, and predicted_unit_data.csv, which has columns needed for the next function we'll be writing for this method.

[shilohd]

Looks great!